METHOD FOR IMPROVING SIGNAL QUALITY OF A DIGITAL SIGNAL BEING PROCESSED IN A LINEAR DEVICE AND APPARATUS USING THE SAME

Abstract

The present invention relates to a method for processing a digital signal through a linear device. The digital signal makes a transition from a first level to a second level. The method comprises pre-emphasizing the digital signal before/after processing it by the linear device. Pre-emphasizing the digital signal includes: pre-emphasizing the digital signal by applying an undershoot to the first level before the transition, when the first level is lower than the second level; and/or pre-emphasizing the digital signal by applying an overshoot to the first level before the transition, when the first level is higher than the second level. The present invention also relates to an apparatus using the above method.

Claims

1. A method for processing a digital signal through a linear device, the digital signal making a first transition from a first level to a second level, the method comprising: pre-emphasizing the digital signal before/after processing it by the linear device; characterized in that pre-emphasizing the digital signal includes: pre-emphasizing the digital signal by applying an undershoot to the first level before the first transition, when the first level is lower than the second level; and/or pre-emphasizing the digital signal by applying an overshoot to the first level before the first transition, when the first level is higher than the second level.

2. The method according to claim 1, wherein the digital signal further makes a second transition, following the first transition, from the second level to a third level, and the pre-emphasizing the digital signal further includes: pre-emphasizing the digital signal by applying an overshoot to the second level before the second transition, when the second level is higher than the third level; and/or pre-emphasizing the digital signal by applying an undershoot to the second level before the second transition, when the second level is lower than the third level.

3. The method according to claim 1 or 2, wherein the undershoot applied to the first level is immediately before the first transition, or the undershoot applied to the second level is immediately before the second transition, or the overshoot applied to the second level is immediately before the second transition, or the overshoot applied to the first level is immediately before the first transition.

4. The method according to any of claims 1 to 3, wherein the linear device has a resonance frequency, or the linear device is described by a Laplace-filter having more than one pole.

5. Apparatus for processing a digital signal through a linear device, the digital signal making a first transition from a first level to a second level, the apparatus comprising: the linear device (402); and a pre-emphasis circuit/driver (401) adapted to pre-emphasize the digital signal before processing it by the linear device (402); characterized in that the pre-emphasis circuit/driver (401) is adapted to pre-emphasize the digital signal by applying an undershoot to the first level before the first transition, when the first level is lower than the second level; and/or to pre-emphasize the digital signal by applying an overshoot to the first level before the first transition, when the first level is higher than the second level.

6. The apparatus according to claim 5, wherein the digital signal further makes a second transition, following the first transition, from the second level to a third level, and the pre-emphasis circuit/driver (401) is further adapted to pre-emphasize the digital signal by applying an overshoot to the second level before the second transition, when the second level is higher than the third level; and/or to pre-emphasize the digital signal by applying an undershoot to the second level before the second transition, when the second level is lower than the third level.

7. The apparatus according to claim 5 or 6, wherein the undershoot applied to the first level is immediately before the first transition, or the undershoot applied to the second level is immediately before the second transition, or the overshoot applied to the second level is immediately before the second transition, or the overshoot applied to the first level is immediately before the first transition.

8. The apparatus according to any one of claims 5 to 7, wherein the linear device (402) has a resonance frequency, or the linear device (402) is described by a Laplace-filter having more than one pole.

9. The apparatus according to any of claims 5 to 8, wherein the input of the linear device (402) is connected to the output of the pre-emphasis circuit/driver (401).

10. Optical receiver comprising: a photodiode (605) for converting an optical digital signal into an electric digital signal; and a first apparatus according to any of claims 5 to 9, wherein the pre-emphasis circuit (601) of the first apparatus is adapted to pre-emphasize the electric digital signal output by the photodiode (605), and the linear device of the first apparatus is a transimpedance amplifier (606) adapted to receive the digital signal pre-emphasized by the pre-emphasis circuit (601).

11. Optical receiver according to claim 10, wherein the pre-emphasis circuit (601) and the transimpedance amplifier of the first apparatus are integrated in one device (606).

12. Communication system comprising: an optical transmitter (708); a photodiode (705); an optical link (707) interconnecting the optical transmitter (708) and the photodiode (705); and the apparatus according to any of claims 5 to 9, wherein the pre-emphasis driver (701) of the apparatus is adapted to receive an electric digital signal, to generate a pre-emphasized electric digital signal based on the received electric digital signal, and to output the pre-emphasized electric digital signal, the optical transmitter (708) is adapted to receive the pre-emphasized electric digital signal, to generate an optical digital signal based on the received pre-emphasized electric digital signal, and to transmit the optical digital signal to the photodiode (705) via the optical link (707), the photodiode (705) is adapted to receive the optical digital signal via the optical link (707), to convert the received optical digital signal into a converted electrical digital signal, and to output the converted electrical digital signal, and the linear device of the apparatus is an amplifier (706), particularly a transimpedance amplifier, adapted to receive the converted electrical digital signal from the photodiode (705).

13. Communication system according to claim 12, wherein the optical transmitter (708) includes an amplifier and a laser device (709), and the pre-emphasis driver (701) of the apparatus and the amplifier of the optical transmitter (708) are integrated in one device.

14. Communication system, comprising: an optical transmitter (708); an optical receiver according to claim 11 or 12; an optical link (707) interconnecting the optical transmitter (708) and the optical receiver; and a second apparatus according to any of claims 5 to 9, wherein the pre-emphasis driver (701) of the second apparatus is adapted to receive an electric digital signal, to generate a pre-emphasized electric digital signal based on the received electric digital signal, and to output the pre-emphasized electric digital signal, the optical transmitter (708) is adapted to receive the pre-emphasized electric digital signal output by the pre-emphasis driver (701) of the second apparatus, to generate an optical digital signal based on the received pre-emphasized electric digital signal, and to transmit the optical digital signal to the optical receiver via the optical link (707), and the linear device of the second apparatus is, for instance, the transimpedance amplifier of the optical receiver according to claim 11 or 12.

15. Communication system according to claim 14, wherein the optical transmitter (708) includes an amplifier and a laser device (709), and the pre-emphasis driver (701) of the second apparatus and the amplifier of the optical transmitter (708) are integrated in one device.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 shows a schematic of an optical receiver for converting an optical signal into an electrical digital signal, used in an optical link of a conventional communication system;

[0012] FIG. 2 illustrates the effect of post-transition pre-emphasis signal processing on a rectangular pulse signal;

[0013] FIG. 3 illustrates the response of the transimpedance amplifier shown in FIG. 1 on a rectangular pulse signal applied at its inputs;

[0014] FIG. 4 shows a schematic of an apparatus for processing a digital signal in accordance with a first embodiment of the present invention;

[0015] FIG. 5a shows the shape of a positive binary pulse input to the pre-emphasis driver of the apparatus for processing a digital signal according to the first embodiment of the present invention and the shape of the pulse output by the same in response to the positive binary input pulse;

[0016] FIG. 5b shows the shape of a negative binary pulse input to the pre-emphasis driver of the apparatus for processing a digital signal according to the first embodiment of the present invention and the shape of the pulse output by the same in response to the negative binary input pulse;

[0017] FIG. 6 shows a schematic of an optical receiver in accordance with a second embodiment of the present invention;

[0018] FIG. 7 shows a schematic of a communication system in accordance with a third embodiment of the present invention.

[0019] Referring now to FIG. 4, an apparatus for processing a digital signal according to the first embodiment of the present invention is shown. The apparatus for processing a digital signal 400 according to the first embodiment of the present invention comprises a pre-emphasis driver 401 and a linear device 402. The pre-emphasis driver 401 is adapted to receive at its input an electrical digital signal including at least one (binary) pulse, to pre-emphasize/peak the received electrical digital signal, and to output a pre-emphasized electrical digital signal. The linear device 402 is adapted to receive at its input an electrical digital signal pre-emphasized by the pre-emphasis driver 401, to process the received signal, and to output the processed digital signal to other devices for further processing (not shown in FIG. 4). The linear device 402 can be a device/system having a resonance frequency, or a device/system described by a Laplace-filter having more than one pole.

[0020] The pre-emphasis driver 401 of the apparatus for processing a digital signal according to the first embodiment of the present invention is adapted to emphasis/peak a (binary) signal level of a digital signal immediately before the transition from one binary signal level to the other binary signal level. In particular, the pre-emphasis driver 401 is adapted to pre-emphasize the digital signal by applying an undershoot to the first level immediately before the transition, when the first level is lower than the second level (i.e. at a positive transition), and to pre-emphasize the digital signal by applying an overshoot to the first level immediately before the first transition, when the first level is higher than the second level (i.e. at a negative transition). Therefore, pre-emphasis driver 401 is denoted in the following as pre-transition pre-emphasis driver 401.

[0021] Referring now to FIGS. 5a and 5b, the effect of the pre-transition pre-emphasis driver 401 on the received electrical digital signal is explained:

[0022] The curve 501 in FIG. 5a represents the shape of a positive pulse of a digital (binary) signal input to the pre-transition pre-emphasis driver 401. This pulse makes a first, positive transition from a lower level (“0”-level) to an upper level (“1”-level), remains approximately constant at the upper level for the duration of the pulse, and thereafter makes a second, negative transition from the upper level to the lower level. The curve 502 represents the shape of the signal output by the pre-transition pre-emphasis circuit 401 in response to the pulse 501. This curve exhibits an undershoot immediately before (or next to) the positive transition from the lower level to the upper level and an overshoot immediately before (or next to) the negative transition from the upper level to the lower level. Particularly, the curve 502 undershoots the lower level by the undershoot 503, makes a first transition from the lower level to the upper level, stays approximately constant at the upper level, overshoots the upper level by the overshoot 504, makes a second transition from the upper level to the lower level, and then stays approximately constant to the lower level.

[0023] The curve 511 in FIG. 5b represents the shape of a negative pulse of a digital (binary) signal input to the pre-transition pre-emphasis driver 401. This pulse starts at an upper level (“0”-level), makes a first, negative transition from the upper level to a lower level (“−1”-level), remains approximately constant at the lower level during the pulse length, and thereafter makes a second, positive transition from the lower level to the upper level. Curve 512 in FIG. 5b represents the shape of the signal output by the pre-transition pre-emphasis driver 401 in response to the pulse 511. This curve exhibits an overshoot immediately before (or next to) the negative transition from the upper level to the lower level and an undershoot immediately before (or next to) the positive transition from the lower level to the upper level. Particularly, the curve 512 overshoots the upper level (“0”-level) by the overshoot 513, makes a first transition from the upper level to the lower level (“−1”-level), stays approximately constant at the lower level, undershoots the lower level (“−1”-level) by the undershoot 514, makes a second transition from the lower level to the upper level, and then stays approximately constant at the upper level.

[0024] For achieving the effects shown in FIGS. 5a and 5b, the pre-transition pre-emphasis driver 401, for instance, is adapted to split the received digital electric signal into a main path signal and a pre-emphasis path signal, to delay the main path signal about the length of a quarter of a pulse length, to invert and attenuate the pre-emphasis path signal, and to sum up the delayed main path signal and the attenuated inverted pre-emphasis path signal.

[0025] In FIG. 5a, which shows the output signal of the pre-emphasis driver 401 in response to the input pulse 501, the undershoot applied to the “0”-level is immediately before (or next to) the positive transition of the pulse 502, and the overshoot applied to the “1”-level is immediately before the negative transition of the pulse 502. However, the present invention is not limited to this case, but also covers the following: i) only an undershoot is applied to the “0”-level immediately before the positive transition of pulse 502, but no overshoot is applied to the “1”-level immediately before the negative transition of pulse 502; and ii) only an overshoot is applied to the “1”-level immediately before the negative transition of pulse 502, but no undershoot is applied to the “0”-level immediately before the positive transition of pulse 502.

[0026] Furthermore, it is not mandatory for the present invention that the undershoot applied to the lower level is immediately before the positive transition of pulse 502 and that the overshoot applied to the upper level is immediately before the negative transition of pulse 502. It is rather important that the undershoot applied to the lower level of pulse 502 is closer to the positive transition of pulse 502 than to the negative transition of a pulse preceding pulse 502, and that the overshoot applied to the upper level of pulse 502 is closer to the negative transition of pulse 502 than to the positive transition of pulse 502.

[0027] In FIG. 5b, which shows the output signal of the pre-emphasis driver 401 in response to the input pulse 511, the overshoot applied to the “0”-level is immediately before (or next to) the negative transition of pulse 512, and the undershoot applied to the “−1”-level is immediately before (or next to) the positive transition of pulse 512. However, the present invention is not limited to this case, but also covers the following: i) only an overshoot is applied to the “0”-level immediately before the negative transition of pulse 512, but no undershoot is applied to the “−1”-level immediately before the positive transition of pulse 512; and ii) only an undershoot is applied to the “−1”-level immediately before the positive transition of pulse 512, but no overshoot is applied to the “0”-level immediately before the negative transition of pulse 512.

[0028] Furthermore, it is not mandatory for the present invention that the overshoot applied to the upper level is immediately before the negative transition of pulse 512 and that the undershoot applied to the lower level is immediately before the positive transition of pulse 512. It is rather important that the overshoot applied to the upper level of pulse 512 is closer to the negative transition of pulse 512 than to the positive transition of a pulse preceding pulse 512, and that the undershoot applied to the lower level of pulse 512 is closer to the positive transition of pulse 512 than to the negative transition of pulse 512.

[0029] In FIGS. 5a and 5b only pulses making a transition from a first level to a second level and vice versa are shown. However, the present invention is not limited to a binary (two-level) digital signal, but is also applicable to a multi-level digital signal having more than two levels, making transitions between any two levels of the multi-level digital signal, and making an arbitrary number of (positive and/or negative) transitions between two levels of the multi-level digital signal.

[0030] In the apparatus for processing a digital signal according to the first embodiment of the present invention, the input of the linear device 402 is connected (directly) to the output of the pre-transition pre-emphasis driver 401, so that the linear device 402 receives at its input the pre-emphasized electrical digital signal output by the pre-transition pre-emphasis driver 401. However, the apparatus for processing a digital signal according to the present invention can have one or more digital signal processing units interposed between the output of the pre-transition pre-emphasis driver 401 and the input of the linear device 402, so that the linear device 402 receives at its input a pre-emphasized electrical digital signal that has been further processed by the one or more digital signal processing units interposed between pre-transition pre-emphasis driver 401 and linear device 402. It is important for the present invention that the digital signal is pre-emphasized by the pre-transition pre-emphasis driver 401 before it is processed by the linear device 402.

[0031] The pre-transition pre-emphasis driver 401 compensates/reduces the effects caused by bandwidth limitation in the digital signal output by the linear device 402. However, the quality of the digital signal output by the linear device 402 of the first embodiment of the present invention is better than the quality of the digital signal output by the linear device 102 of the optical receiver 100 shown in FIG. 1, because the pre-transition pre-emphasis driver 401 of the first embodiment of the present invention does not enhance ringing in the outputted digital signal. This will be demonstrated later.

[0032] Referring now to FIG. 6, an optical receiver in accordance with a second embodiment of the present invention is described. The optical receiver 600 in accordance with the second embodiment of the present invention comprises a photodiode 605, for instance, a positive intrinsic negative diode, and a transimpedance amplifier with a pre-transition pre-emphasis circuit 601. The pre-transition pre-emphasis circuit 601 and the transimpedance amplifier are integrated in one device 606, which is connected to the photodiode 605 by means of interconnects 603. The transimpedance amplifier of the optical receiver according to the second embodiment corresponds to the linear device 402 of the apparatus according to the first embodiment. The photodiode 605 receives an optical digital signal, converts the received optical digital signal into an electric digital signal, and outputs the electric digital signal to the interconnects 603. The transimpedance amplifier with the pre-transition pre-emphasis circuit receives the electric digital signal output by the photodiode 605 via the interconnects 603, and outputs an electric signal which is compensated for the bandwidth limitation and ringing by means of the pre-transition pre-emphasis circuit 601.

[0033] The effect/response of the pre-transition pre-emphasis circuit 601 on a received electric digital signal is the same as the effect/response of the pre-transition pre-emphasis driver 401 used in the first embodiment. Also, the description of the pre-transition pre-emphasis driver 401 of the first embodiment applies to the pre-transition pre-emphasis circuit 601 of the second embodiment. Therefore, a detailed description of the pre-transition pre-emphasis circuit 601 is omitted.

TABLE-US-00001 TABLE 1 Without Post-transition Pre-transition Parameter pre-emphasis pre-emphasis pre-emphasis Eye height 80 87 97 Rise/Fall time [ps] 24 19 14 Deterministic 3.2 5.7 4.0 jitter [ps] Overshoot/Undershoot 12 34 22

[0034] The effect of the pre-transition pre-emphasis circuit 601 on the electric digital signal output by the transimpedance amplifier becomes evident from Table 1. The table indicates parameters of (positive) pulses output by the transimpedance amplifier of an optical receiver that: i) does not apply pre-emphasis signal processing to the electric digital signal output by the PIN; ii) applies post-transition pre-emphasis signal processing, as shown in FIG. 2, to the electric digital signal output by the PIN; and iii) applies pre-transition pre-emphasis signal processing, as shown in FIG. 5a, to the electric digital signal output by the PIN. The pulses are output in response to rectangular pulses of an optical digital signal having a transmission rate of 25 Gbps. At the input of the positive intrinsic negative diode of the optical receiver, the rise time and fall time of a pulse of the optical digital signal is 21 ps. The parameters given in Table 1 are: eye height of the eye diagram of the electric digital signal output by the transimpedance amplifier, rise/fall time of the pulses of the electric digital signal output by the transimpedance amplifier, deterministic jitter derived from the eye diagram, and overshoot/undershoot of the pulses of the electric digital signal output by the transimpedance amplifier. Rise and fall time are determined by the 20%-level and 80%-level of the slope of the eye diagram.

[0035] Table 1 shows that pre-transition pre-emphasis signal processing leads to an opening of the eye diagram. This opening is greater than the opening caused by post-transition pre-emphasis signal processing. The deterministic jitter induced by pre-transition pre-emphasis signal processing is lower than the deterministic jitter induced by post-transition pre-emphasis signal processing, and only slightly increased compared to the deterministic jitter of a digital signal that has not been subjected to pre-emphasis signal processing.

[0036] As random jitter is proportional to rise/fall time, table 1 also indicates that random jitter of a digital signal that has been subjected to pre-transition pre-emphasis signal processing is lower than random jitter of a digital signal that has been subjected to post-transition pre-emphasis signal processing, and is much lower than random jitter of a digital signal that has not been subjected to pre-emphasis signal processing at all.

[0037] Hence, the present invention advantageously increases the opening of the eye diagram and decreases random jitter without increasing the deterministic jitter significantly.

[0038] Furthermore, table 1 shows that the overshoot/undershoot of a digital signal subjected to pre-transition pre-emphasis signal processing is lower than the overshoot/undershoot of a digital signal subjected to post-transition pre-emphasis signal processing, and is only a little higher than the overshoot/undershoot of a digital signal that has not been subjected to pre-emphasis signal processing at all.

[0039] This is evidence that the present invention compensates the disadvantageous effects caused by bandwidth limitation without enhancing ringing significantly.

[0040] In FIG. 6, the pre-transition pre-emphasis circuit 601 and the transimpedance amplifier are integrated in one device 606. However, the present invention is not limited to this configuration, but also includes configurations, wherein the pre-emphasis circuit 601 and the transimpedance amplifier are separate from each other.

[0041] Referring now to FIG. 7, a communication system in accordance with a third embodiment of the present invention is shown. The communication system according to the third embodiment of the present invention comprises a pre-transition pre-emphasis circuit 701, an optical transmitter 708, an optical receiver, and an optical fiber 707 interconnecting the optical transmitter 708 and the optical receiver.

[0042] The pre-transition pre-emphasis circuit 701 receives an electric digital signal, pre-emphasizes the received electric digital signal, and outputs the pre-emphasized electric digital signal to the optical transmitter 708. The effect/response of the pre-transition pre-emphasis circuit 701 on an electric digital signal is the same as the effect/response of the pre-transition pre-emphasis driver 401 used in the first embodiment. Also, the description relating to the pre-transition pre-emphasis driver 401 of the first embodiment applies to the pre-transition pre-emphasis circuit 701 of the third embodiment. Therefore, a detailed description of the pre-transition pre-emphasis circuit 701 is omitted.

[0043] The optical transmitter 708, which includes an amplifier and a laser device, for instance, a vertical-cavity surface-emitting laser 709, receives the pre-emphasized electric digital signal output by the pre-emphasis circuit 701, generates an optical digital signal corresponding to the received pre-emphasized electric digital signal by means of the vertical-cavity surface-emitting laser 709, and transmits the generated optical digital signal to the optical receiver via the optical fiber 707.

[0044] The optical receiver includes a photodiode 705, for instance, a positive intrinsic negative diode, and a transimpedance amplifier 706 connected to the photodiode 705 by means of interconnects 703. The photodiode 705 receives an optical digital signal from the optical fiber 707, converts the received optical digital signal into an electric digital signal, and outputs the electric digital signal to the interconnects 703. The transimpedance amplifier 706 receives the electric digital signal output by the photodiode 705 via interconnects 703.

[0045] As the optical transmitter 708 receives the pre-emphasized electric digital signal output by the pre-emphasis circuit 701, a pulse of the optical digital signal generated by the vertical-cavity surface-emitting laser 709 and transmitted to the optical receiver via the optical fiber 707, has the shape of the curve 502 shown in FIG. 5a. Also, the electrical pulse input to the transimpedance amplifier 706 has the shape of the curve 502 given in FIG. 5a, as the shape of the electrical pulse output by the photodiode 705 corresponds to the shape of the optical pulse received by the photodiode 705. Therefore, a pulse of the electric digital signal output by the transimpedance amplifier 706 shows the same advantageous parameters as the output pulse of the transimpedance amplifier 606 of the second embodiment.

[0046] In FIG. 7, the pre-transition pre-emphasis circuit 701 and the optical transmitter 708 are separate from each other. However, the present invention is not limited to this configuration, but also includes optical transmitters, wherein the pre-transition pre-emphasis circuit 701 is included in the optical transmitter 708. Advantageously, the pre-transition pre-emphasis circuit 701 and the amplifier of the optical transmitter 708 are integrated in one device.

[0047] In the communication system of FIG. 7, the pre-transition pre-emphasis circuit 701 for pre-emphasizing a digital signal to be communicated from the optical transmitter 708 to the optical receiver is located at the transmitter's side. However, the present invention is not limited to this embodiment, but also relates to a communication system, wherein the digital signal communicated between optical transmitter and optical receiver is pre-emphasized by a pre-transition pre-emphasis circuit located at the optical receiver's side, for instance, by the optical receiver 600 according to the second embodiment.

[0048] Also, the present invention relates to a communication system comprising: the optical receiver 600 according to the second embodiment, which includes a first pre-transition pre-emphasis circuit, and a second pre-transition pre-emphasis circuit 701 located at the optical transmitter's side. Preferably, the second pre-transition pre-emphasis circuit and the amplifier of the optical transmitter are integrated in one device. In this communication system, the digital signal communicated between optical transmitter and optical receiver is pre-emphasized according to the present invention on the transmitter's and on the receiver's side.

[0049] The present invention compensates/reduces the negative effects caused by bandwidth limitation without enhancing ringing. Also, the present invention increases the opening of the eye diagram and decreases random jitter without increasing the deterministic jitter significantly. Therefore, the present invention is especially suited in communication systems having high data transmission rates, such as 25 Gbps.

REFERENCE NUMERALS

[0050]

TABLE-US-00002 Reference Numeral Description 100 Conventional optical receiver 101 Photodiode, e.g. positive intrinsic negative diode (PIN) 102 Transimpedance amplifier (TIA) 103 PIN-TIA interconnects 201 Shape of an rectangular pulse subjected to post-transition pre- emphasis signal processing 202 Overshoot of an rectangular pulse subjected to post-transition pre- emphasis signal processing 203 Undershoot of an rectangular pulse subjected to post-transition pre- emphasis signal processing 301 Response of a conventional transimpedance amplifier on a rectangular pulse signal applied at its inputs 302 Ringing 400 Apparatus for processing a digital signal according to the invention 401, 601, 701 Pre-transition pre-emphasis driver/circuit for an electrical digital signal 402 Linear device 501, 511 Digital pulse input to the pre-transition pre-emphasis driver/circuit 502, 512 Digital pulse output by the pre-transition pre-emphasis driver/circuit 503, 514 Undershoot of a level of the digital pulse output by the pre-transition pre-emphasis driver/circuit 504, 513 Overshoot of a level of the digital pulse output by the pre-transition pre-emphasis driver/circuit 600 Optical receiver according to the invention 603, 703 PIN-TIA interconnects 605, 705 Photodiode 606, 706 Transimpedance amplifier 700 Communication system according to the invention 707 Optical link 708 Optical transmitter 709 Vertical-cavity surface-emitting laser