Wavelength tunable bidirectional optical wireless communication system based on self-injection lock

Abstract

A wavelength tunable bidirectional optical wireless communication system based on self-injection lock includes one optical node and multiple optical terminals, wherein the optical node consists of a tunable filter and a self-injection lock system to replace the conventional optical amplifier while achieving an amplified optical power, increasing the modulation bandwidth, wavelength adjustment and reducing the linewidth of each wavelength, in a low noise criteria. The optical terminal is composed by a modulated retroreflector to achieve the purpose of lightweight and low power consumption.

Claims

1. A wavelength tunable bidirectional optical wireless communication system based on self-injection lock, comprising: an optical node, comprising: an optical transceiver capable of transmitting a set of unmodulated incident light beams, wherein the unmodulated incident light beams have specific wavelength range; a tunable bandpass filter arranged on the path of the incident light beams for the specific wavelength range in the incident light beams to pass through; a reflector arranged on the path of the incident light beams; and a beamsplitter arranged on the path of the incident light beams to split the incident light beams which pass through the tunable bandpass filter into a first split beam and a second split beam, wherein the first split beam passes through the beamsplitter and heads to the reflector, and the reflector reflects light beams to the beamsplitter which splits reflected light beams into a third split beam and a fourth split beam, wherein the third split beam passes through the beamsplitter and injects into the optical transceiver and the fourth split beam heads to an optical receiver and is received by the optical receiver; and an optical terminal, comprising: a passive retroreflector for passively reflecting incident light parallel to path of original incident light; and an optical modulator arranged on one side of the passive retroreflector, wherein the optical modulator outputs a reversing light beam with signal after the second split beam passes through the passive retroreflector and enters the optical modulator, then the reversing light beam passes through the passive retroreflector and is parallel to the path of the second split beam, so that the reversing light beam is reflected to the optical node and received by the optical receiver.

2. The optical wireless communication system of claim 1, wherein there is a collimator between the optical transceiver and the tunable bandpass filter to enable the path of the incident light beams transmitted from the optical transceiver to travel in parallel.

3. The optical wireless communication system of claim 1, wherein there is a polarizer between the optical transceiver and the tunable bandpass filter to control the polarization direction of the incident light beams transmitted from the optical transceiver.

4. The optical wireless communication system of claim 1, wherein the reflector is a high reflectivity mirror.

5. The optical wireless communication system of claim 1, wherein the optical modulator is a multiple quantum well modulator, a Liquid Crystal Spatial Light Modulator (LC-SLM), a Liquid Crystal on Silicon Spatial Light Modulator (LCoS-SLM), a microelectromechanical system spatial light modulator or a Lithium Niobate Electro-Optics Modulator.

6. The optical wireless communication system of claim 1, wherein the passive retroreflector is a cat-eye lens group or a corner cube retroreflector.

7. The optical wireless communication system of claim 1, wherein the reversing light beam enters the beamsplitter and is split into a fifth split beam and a sixth split beam, the fifth split beam passes through the beamsplitter and is injected into the optical receiver, and the sixth split beam is directed towards the optical transceiver by the beamsplitter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the overall framework of the wavelength tunable bidirectional optical wireless communication system based on self-injection lock of the present invention.

(2) FIG. 2 is a realistic schematic diagram of the wavelength tunable bidirectional optical wireless communication system based on self-injection lock of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(3) Other technical contents, aspects and effects in relation to the present invention can be clearly appreciated through the detailed descriptions concerning the preferred embodiments of the present invention in conjunction with the appended drawings.

(4) As shown in FIG. 1 and FIG. 2, the wavelength tunable bidirectional optical wireless communication system based on self-injection lock of the present invention comprises an optical node 1 and an optical terminal 2, wherein the optical node 1 comprises:

(5) (1) an optical transceiver 11 capable of transmitting a set of unmodulated incident light beams 111, wherein the incident light beams 111 have specific wavelength range. In this embodiment of the present invention, the optical transceiver 11 utilizes wideband laser such as Fabry-Pérot Laser Diode (FP-LD). The laser source of the optical transceiver 11 may be served as the master laser and the slave laser simultaneously. The transmitted laser is self-injected to the original laser after reflection to amplify and re-transmit the light.
(2) a collimator 12 is used to enable the path of the incident light beams 111 transmitted from the transceiver 11 to travel in parallel.
(3) a polarizer 13 is used to control the polarization direction of the incident light beams 111 transmitted from the optical transceiver 11.
(4) a tunable bandpass filter 14 is used to filter the incident light beams 111 for the specific wavelength range in the incident light beams 111 to pass through.
(5) a reflector 16 is arranged on the path of the incident light beams 111, wherein the reflector 16 is a high reflectivity mirror.
(6) a beamsplitter 15 is arranged on the path of the incident light beams 111 to allow the incident light beams 111 which pass through the tunable bandpass filter 14 to split into different paths. The path description is as follows:

(6) (a) After the incident light beams 111 contact the beamsplitter 15, the incident light beams 111 are split into a first split beam 151 and a second split beam 152, wherein the first light beam 151 can pass through the beamsplitter 15 and direct to the reflector 16.

(7) (b) The reflector 16 can reflect the light beams 161 to the beamsplitter 15, and the light beams 161 are then split into a third split beam 153 and a fourth split beam 154. The third split beam 153 passes through the beamsplitter 15 and is injected into the optical transceiver 11. The fourth split beam 154 heads to the optical receiver 17 and is received by the optical receiver 17. Furthermore, there is a lens 18 between the beamsplitter 15 and the optical receiver 17.

(8) The optical terminal 2 comprises:

(9) (1) a passive retroreflector 21 is used to passively reflect the incident light parallel to the original incident light path. In this embodiment of the present invention, the passive retroreflector 21 is a cat-eye system (also called a cat-eye lens group) or a corner cube retroreflector.
(2) an optical modulator 22 is arranged on one side of the passive retroreflector 21. The optical modulator 22 is a multiple quantum well modulator (MQW), a Liquid Crystal Spatial Light Modulator (LC-SLM), a Liquid Crystal on Silicon Spatial Light Modulator (LCoS-SLM), a microelectromechanical system (MEMS) spatial light modulator or a Lithium Niobate Electro-Optics Modulator, wherein the optical path through the optical modulator 22 is described as follows:

(10) (a) After the second split beam 152 passes through the passive retroreflector 21 and enters the optical modulator 22, the optical modulator 22 can modulate a reversing light beam 221 with signal, and then the reversing light beam 221 passes through the passive retroreflector 21 and is parallel to the path of the second split beam 152, so that the reversing light beam 221 is returned to the optical node 1 and received by the optical receiver 17.

(11) (b) After the reversing light beam 221 enters the beamsplitter 15 and is split into a fifth split beam 155 and a sixth split beam 156. The fifth split beam 155 passes through the beamsplitter 15 and is injected into the optical receiver 17, and the sixth split beam 156 is directed towards the optical transceiver 11 by the beamsplitter 15.

(12) As described above, the present invention intends to utilize a broadband laser such as Fabry-Pérot laser (FP-LD) at the optical node 1 in combination with a tunable bandpass filter 14 and SIL to replace the convention optical amplifier. At the same time, the objectives of amplifying the optical power, increasing the modulation bandwidth, reducing the line width, low noise and adjustable wavelength are achieved.

(13) The advantages of SIL are reduction of linewidth, enhancement of the frequency response, increase of modulation bandwidth, reduction of relative intensity noise (RIN) and nonlinear distortion, and improvement of link gain. It is different from the general free-running laser, SIL has two important parameters. One is the external injection ratio which is defined as:

(14) $\begin{array}{cc}{R}_{inj}=k_c\frac{A_{\mathrm{inj}}}{A_{fr}}& \mathrm{Formula}\left(1\right)\end{array}$
wherein k.sub.c is a coupling coefficient, A.sub.fr is a steady state field magnitude of the free-running slave laser, A.sub.inj is the injection field magnitude of the injected master laser. Since the optical power is proportional to the square of the field intensity (P∝A.sup.2), the formula (1) can be rewritten as dB form for the convenience of measurement.

(15) $\begin{array}{cc}{R}_{\mathrm{inj}}=\frac{P_{\mathrm{inj}}}{P_{fr}}& \mathrm{Formula}\left(2\right)\end{array}$
Another important parameter is the detuning frequency, which is usually expressed as frequency detuning (GHz) or wavelength detuning (pm) and defined as:
Δω=ω.sub.master−ω.sub.slave  Formula (3)
Wherein ω.sub.master is the frequency of the master laser, and ω.sub.slave is the frequency of the slave laser. The direct modulation of the slave laser is not symmetrical. Therefore, the positive or negative value of the detuning frequency has different influence on the slave laser. Negative detuning frequency can achieve stable injection locking. On the contrary, positive detuning frequency usually leads to more complicated dynamic work, such as P1 dynamic state. The small modulation response of the injection locking and each parameter can be derived from the rate equation formula (4) to formula (6) as follows:

(16) $\begin{array}{cc}\frac{\mathrm{dA}\left(t\right)}{\mathrm{dt}}=\frac{1}{2}\Gamma \mathrm{gA}\left(t\right)-\frac{1}{2}{\gamma }_{c}A\left(t\right)+K_c{A}_{inj}\cos \left(\varphi \left(t\right)\right)& \mathrm{Formula}\left(4\right)\\ \frac{d\varphi \left(t\right)}{\mathrm{dt}}=\frac{\alpha }{2}\Gamma g-\frac{\alpha }{2}{\gamma }_c-\Delta \omega -k_c\frac{A_{\mathrm{inj}}}{A\left(t\right)}\sin \left(\varphi \left(t\right)\right)& \mathrm{Formula}\left(5\right)\\ \frac{\mathrm{dN}\left(t\right)}{\mathrm{dt}}=J\left(t\right)-{\gamma }_{s}N\left(t\right)-{\mathrm{gA}}^2\left(t\right)& \mathrm{Formula}\left(6\right)\end{array}$
Wherein A(t) and N(t) are the electrical field intensity and the carrier density of the slave laser respectively, ϕ(t) is the phase difference between the master laser and the slave laser, a is the linewidth gain parameter of the slave laser, J(t) is the bias current density, γ.sub.s is the spontaneous carrier relaxation rate, γ.sub.c is the intra-cavity photon attenuation rate and defined as formula (7), τ.sub.p is the photon lifetime, Γ is the optical confinement factor, g is the gain constant and g.sub.th is the threshold gain.

(17) $\begin{array}{cc}{\gamma }_c=\Gamma g_{th}=\frac{1}{{\tau }_p}& \mathrm{Formula}\left(7\right)\end{array}$
Formula (8) to Formula (10) can be derived from Formula (4) to Formula (6):

(18) $\begin{array}{cc}{S}_0=\frac{s_{\mathrm{fr}}-\left(\frac{{\gamma }_N}{{\gamma }_p}\right)\Delta N_0}{1+\left(\frac{g\Delta N_0}{{\gamma }_p}\right)}& \mathrm{Formula}\left(8\right)\\ {\varphi }_0={\sin }^{-1}\left(-\frac{\Delta {\omega }_{\mathrm{inj}}}{k_c\sqrt{1+{\alpha }^2}}\sqrt{\frac{S_0}{S_{\mathrm{inj}}}}\right)-{\tan }^{-1}\alpha & \mathrm{Formula}\left(9\right)\\ \Delta N_0=-\frac{2k}{g}\sqrt{\frac{S_{\mathrm{inj}}}{S_0}}\cos {\varphi }_0& \mathrm{Formula}\left(10\right)\end{array}$
Wherein S.sub.0 is the optimizing steady state photon density under injection locking, ΔN.sub.0≡N.sub.0−N.sub.th is the carrier density variation, N.sub.0 is the optimizing steady state carrier density under injection locking, N.sub.th is the steady state carrier density under free running condition, and the resonant frequency ω.sub.R can be written as:

(19) $\begin{array}{cc}{\omega }_R^2\approx {\omega }_{R0}^2+\Delta {\omega }_R^2=.Math.-\frac{\alpha }{2}g\left(N_0-N_{th}\right)+\mathrm{\Delta \omega }.Math.& \mathrm{Formula}\left(11\right)\\ {\mathrm{\Delta \omega }}_R\equiv .Math.k_c\sqrt{\frac{S_{\mathrm{inj}}}{S_0}}\sin \left({\varphi }_0\right).Math.& \mathrm{Formula}\left(12\right)\end{array}$
Wherein ω.sub.R0 is the relaxation oscillation frequency of the free running laser, Δω.sub.R.sup.2 is the frequency enhancement term. The SIL improves the modulation bandwidth has been proved and verified by the experiments. The curve can be obtained by analyzing the poles of the direct modulation frequency response intensity, and the poles can also be derived from the frequency response.

(20) $\begin{array}{cc}H\left(\omega \right)=\frac{ZM}{\left(j\omega +{\omega }_p\right)\left(j\omega -j{\omega }_R+\frac{1}{2}\gamma \right)\left(j\omega +j{\omega }_R+\frac{1}{2}\gamma \right)}& \mathrm{Formula}\left(13\right)\\ \gamma \approx {\gamma }_0-g\left(N_0-N_{th}\right)={\gamma }_0+2k\sqrt{\frac{S_{\mathrm{inj}}}{S_0}}\cos \left({\varphi }_0\right)& \mathrm{Formula}\left(14\right)\end{array}$
Wherein γ is the damping ratio, the first pole of the frequency response −ω.sub.p determines the response of the low pass filter, which affects the 3-dB frequency response. External resonant cavity is the distance from LD to the high reflectivity mirror. In formula (15), Δλ is the wavelength interval in FP mode, λ.sub.c is the central wavelength of FP-LD, n is the medium of the external resonant cavity, L.sub.ext is the length of the external resonant cavity. We can conclude that the distance of the external resonant cavity has an effect on the modulation bandwidth, linewidth and wavelength interval in FP mode.

(21) $\begin{array}{cc}\Delta \lambda =\frac{{\lambda }_c^2}{2n{L}_{\mathrm{ext}}}& \mathrm{Formula}\left(15\right)\end{array}$

(22) As mentioned above, the present invention uses the cat-eye system as the embodiment of the passive retroreflector 21, wherein the cat-eye system is composed of multiple lens and a mirror to allow the reflected light parallel to the original incident light passively. Its important parameters are front-end optical gain, FOV, modulation efficiency, modulator radius, and theoretical optical signal-to-noise ratio, which can be derived from formula (16) to formula (20) respectively:

(23) $\begin{array}{cc}{G}_{\mathrm{MRR}}={\left[\frac{\pi D_{\mathrm{retro}}}{\lambda }\right]}^{4}S& \mathrm{Formula}\left(16\right)\\ \mathrm{AFOV}\left(°\right)=2\times {\tan }^{-1}\left(\frac{h}{2f}\right)& \mathrm{Formula}\left(17\right)\\ M=e^{-{\alpha }_{\mathrm{On}}}-e^{-{\alpha }_{\mathrm{Off}}}=e^{-{\alpha }_{\mathrm{Off}}}.Math.\left[C_{\mathrm{mod}}-1\right]& \mathrm{Formula}\left(18\right)\\ D_{\mathrm{mod}}=f\#D_{\mathrm{retro}}{\mathrm{FOV}}_{\mathrm{retro}}& \mathrm{Formula}\left(19\right)\\ \mathrm{OSNR}=\frac{{\mathrm{MP}}_{\mathrm{Ret}}}{P_{\mathrm{noise}}}& \mathrm{Formula}\left(20\right)\end{array}$
Wherein D.sub.retro is the optical aperture, S is Strehl ratio, h is the modulator diameter, f is the focal length of the cat-eye system, α.sub.On and α.sub.Off are double-pass absorption-length product of the modulator when it is on and off, C.sub.mod is the optical contrast of the modulator and P.sub.noise is the noise equivalent power of the photodetector.

(24) Compared with other conventional technologies, the wavelength tunable bidirectional optical wireless communication system based on self-injection lock provided in the present invention has the advantage of combining self-injection locking (SIL) technique at the optical node 1 and achieving the FSOWC with low energy consumption and high SNR through multiple lenses and passive retroreflectors 21. The advantages of SIL include not only increasing the transmission power and modulation bandwidth, but also reducing the spectral linewidth and consuming less energy than EDFA/SOA. At the same time, the purpose of low cost and flexible wavelength can be achieved by Fabry-Pérot laser and tunable bandpass filter 14.

(25) Although the present invention has been disclosed through the aforementioned embodiments, such illustrations are by no means used to restrict the scope of the present invention. That is, any person having ordinary skill in relevant fields of the present invention can certainly devise any applicable alterations and modifications after having comprehended the aforementioned technical characteristics and embodiments of the present invention within the spirit and scope thereof. Hence, the scope of the present invention to be protected under patent laws should be delineated in accordance with the claims set forth hereunder in the present specification.