Optical pulse source apparatus with nonlinear fibre and operable to reduce the optical pulse frequency of optical output pulses

Abstract

The invention can include an optical pulse source apparatus that includes the nonlinear generation of wavelengths, wherein the optical pulse source can comprise an oscillator for producing optical pulses, the optical pulses having a first wavelength; an optical fiber amplifier for amplifying optical pulses having the first wavelength; a nonlinear optical fiber receiving amplified optical pulses having the first wavelength to nonlinearly produce optical pulses that include wavelengths that are different than the first wavelength; and wherein the optical pulse source is configured so as to be operable to reduce the optical pulse frequency of the nonlinearly produced optical pulses.

Claims

1. An optical pulse source apparatus including nonlinear generation of wavelengths, comprising: an oscillator for producing optical pulses, the optical pulses having a first wavelength; an optical amplifier for amplifying optical pulses having the first wavelength; and a nonlinear optical fiber receiving amplified optical pulses having the first wavelength to nonlinearly produce optical pulses that include wavelengths that are different than the first wavelength; wherein the optical pulse source apparatus varies the optical pulse frequency of the optical pulses prior to receipt thereof by the nonlinear optical fiber so as to selectively control the optical pulse frequency of the resultant nonlinearly produced optical pulses.

2. The optical pulse source apparatus according to claim 1, wherein the optical pulse frequency of the optical pulses is varied by a pulse picker in optical communication with the oscillator and the nonlinear optical fiber.

3. The optical pulse source apparatus according to claim 2, wherein the pulse picker is positioned between the oscillator and the optical amplifier.

4. The optical pulse source apparatus according to claim 1, wherein the optical pulses having the first wavelength are varied prior to being amplified by the optical amplifier.

5. The optical pulse source apparatus according to claim 4, wherein the optical pulse frequency of the optical pulses is varied by a pulse picker in optical communication with the oscillator and the nonlinear optical fiber.

6. The optical pulse source apparatus according to claim 5, wherein the pulse picker is positioned between the oscillator and the optical amplifier.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic representation of a high power short optical pulse source according to a first embodiment of the invention;

(3) FIG. 2 is a schematic representation of a high power short optical pulse source according to a second embodiment of the invention;

(4) FIG. 3 is a schematic representation of a high power short optical pulse source according to a third embodiment of the invention;

(5) FIG. 4 is a schematic representation of a high power short optical pulse source according to a fourth embodiment of the invention;

(6) FIG. 5 is a schematic representation of a high power short optical pulse source according to a fifth embodiment of the invention;

(7) FIG. 6 is a schematic representation of a high power short optical pulse source according to a sixth embodiment of the invention;

(8) FIG. 7 is a schematic representation of a high power short optical pulse source according to a seventh embodiment of the invention;

(9) FIG. 8 is a schematic representation of a high power short optical pulse source according to a eighth embodiment of the invention;

(10) FIG. 9 is a schematic representation of a high power short optical pulse source according to a ninth embodiment of the invention;

(11) FIG. 10 is a schematic representation of a high power short optical pulse source according to a tenth embodiment of the invention; and

(12) FIG. 11 is a schematic representation of a high power short optical pulse source according to an eleventh embodiment of the invention.

DETAILED DESCRIPTION

First Embodiment

(13) Referring to FIG. 1, a first embodiment of the invention provides a high power short optical pulse source 10 comprising an optical signal source in the form of a master oscillator 12 and an Ytterbium doped fiber amplifier (YDFA) preamplifier 14, a pump light source 16, an optical head 18, high power optical fiber amplifier means 20, an optical signal delivery fiber 22, a pump light delivery fiber 24, and a high power optical isolator 26.

(14) The master oscillator 12 comprises a modelocked fiber oscillator operable to output 10 ps pulses at a pulse frequency of 200 MHz. The preamplifier 14 amplifies the pulse average power to approximately 100 mW.

(15) In this example, the master oscillator 12, preamplifier 14, and pump laser 16, together with their associated drive and control electronics, and cooling systems (not shown), are provided within a first enclosure 28 at a first location. Additional optical isolators 30, 32 are provided after the master oscillator 12 and the preamplifier 14 to protect them from back reflections.

(16) The high power fiber amplifier means 20 comprises a 1 m long single-mode, polarisation maintaining, large mode area Ytterbium doped amplifier fiber and a pump signal combiner having a large mode area signal fiber. The high power fiber amplifier means 20 and the high power optical isolator 26 are provided within the optical head 18, which is located at a second location, remote from the first location.

(17) The short, relatively low power optical pulses output from the preamplifier 14 are delivered to the pump signal combiner of the high power fiber amplifier means 20 via the optical signal delivery fiber 22, which comprises a 5 m long polarisation maintaining, single-mode optical fiber; the use of polarisation maintaining fiber prevents any environmental effects disturbing the signal.

(18) The pump light source 16 comprises a fiber pig-tailed multi-emitter pump source operable to generate cw pump light at a wavelength of 915 nm, and an optical power of between 20 W and 40 W.

(19) The pump light is delivered to the pump signal combiner of the high power optical fiber amplifier means 20 through the pump delivery fiber 24, which in this example is a 5 m length of optical fiber having a 105 μm diameter core and a numerical aperture of 0.22. The pump delivery fiber 24 is not sensitive to environmental effects but can, along with signal delivery fiber, be protected within an armoured jacket for industrial applications.

(20) The high power optical fiber amplifier means 20 amplifies the received optical pulses to an average power of approximately 20 W, resulting in a pulse peak power of approximately 10 kW. The overall low nonlinearity of the high power optical pulse source 10 ensures that spectral broadening of the optical pulses is minimised, and the pulses have a final spectral bandwidth of less than 0.5 nm.

(21) The optical head 18, containing the high power optical fiber amplifier means 20 and the isolator 26, has a small footprint and can be positioned at the intended target of the optical pulses. The large, noisy elements of the high power short optical pulse source 10 are thereby provided away from the application site of the pulses, and only the smaller optical head need be located near the intended target.

Second Embodiment

(22) FIG. 2 shows a high power short optical pulse source 40 according to a second embodiment of the invention. The optical pulse source 40 of this embodiment is substantially the same as the optical pulse source 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(23) In this embodiment, a second pump light source 42 is provided at the first location, within the first enclosure. A second pump light delivery fiber 44 is also provided and delivers pump light from the second pump light source 42 to the pump signal combiner of the high power optical fiber amplifier means 20.

(24) It will be appreciated that the high power optical fiber amplifier means 20 may alternatively comprise a two-stage amplifier having a series of two amplifier fibers and pump signal combiners, with an isolator provided between amplifier stages as necessary. Each pump signal source 16, 42 would then pump a separate amplifier stage, the amplifier stages being cascaded to provide increasing signal with balanced gain throughout the cascade.

(25) In most applications, the high power optical fiber amplifier means 20 will only require a single amplifier fiber. However, the number of amplifier fibers (and thus the number of pump light sources) will at least in part be determined by the power of the optical pulses output from the preamplifier 14, the length of signal delivery fiber 22, and the permitted amount of nonlinear effects as determined by the application of the high power short optical pulse source 10.

Third Embodiment

(26) A high power short optical pulse source 50 according to a third embodiment of the invention is shown in FIG. 3. The pulse source 50 according to this embodiment is substantially the same as the pulse source 40 of the previous embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(27) In this embodiment, the pump signal combiner 52 of the high power optical fiber amplifier means 20 is provided at the first location, within the first enclosure 28. The pump delivery fibers 24, 44 and the signal delivery fiber 22 are also wholly contained within the first enclosure 28.

(28) The high power optical fiber amplifier means 20 of this embodiment further comprises a pump signal delivery fiber 54, which comprises the output fiber of the pump signal combiner 52. The pump signal delivery fiber 54 is a polarisation maintaining double clad optical fiber which has a signal-guiding core region and an inner cladding which guides the pump light from the pump sources 16, 42.

(29) The pump signal delivery fiber 54 extends from the pump signal combiner 52, within the first enclosure 28, to the amplifier fiber 56, within the optical head 18. The amplifier fiber 56 is therefore pumped in the co-propagating regime.

Fourth Embodiment

(30) FIG. 4 shows a high power short optical pulse source 60 according to a fourth embodiment of the invention. The pulse source 60 of this embodiment is substantially the same as the pulse source 50 of the previous embodiment, with the following modifications.

(31) In this embodiment, the amplifier fiber 62 comprises a polarisation maintaining double clad amplifier fiber and extends between the pump signal combiner 52 and the optical head 18. The input end of the amplifier fiber 62 is coupled to the output of the pump signal combiner 52 and the output end of the amplifier fiber 62 is located in the optical head 18, where it is coupled to an optical isolator and collimating optics 64. The amplifier fiber 62 therefore provides the dual functions of pulse amplification and pulse delivery to the optical head 18.

(32) Since fiber amplifiers are not 100% efficient (typically 50 to 75% efficiency is achievable), the amplifier fiber 62 will get warm during use. Heat dissipation may be achieved by using an armoured cable to protect the amplifier fiber 62; the void around the amplifier fiber 62 being filled with a suitable heat transfer fluid or paste to ensure good thermal contact between the amplifier fiber 62 and the cable inner surface.

Fifth Embodiment

(33) A high power short optical pulse source 70 according to a fifth embodiment of the invention is shown in FIG. 5. The pulse source 70 of this embodiment is substantially the same as the pulse source 60 of the previous embodiment, with the following modifications.

(34) In this example, the pump signal combiner 52 is provided in the optical head 18, so that a counter-propagating pump regime is used. The pump delivery fibers 24, 44 extend from the pump sources 16, 42 in the first enclosure to the pump signal combiner 52 in the optical head 18.

Sixth Embodiment

(35) A high power short optical pulse source 80 according to a sixth embodiment of the invention is shown in FIG. 6. The optical pulse source 80 is substantially the same as the pulse source 60 of FIG. 4, with the following modifications.

(36) In this example, the amplifier fiber 62 is made to be “under-length”, such that quite a large portion of the pump light delivered to the amplifier fiber is not absorbed within the amplifier fiber, but propagates through to the fiber output, along with the signal pulses. The high power optical fiber amplifier means 20 further comprises a rod amplifier 82, in the form of a Yttrium Aluminium Garnet (YAG) crystal rod, optically coupled to the output of the amplifier fiber 62, to form a hybrid amplifier device. The YAG crystal rod 82 performs a second stage of amplification of the short optical pulses, in a very short piece of material, using the unabsorbed pump light to pump the YAG crystal rod 82. The YAG crystal rod 82 can provide an additional gain of a few dB's while providing very little nonlinear distortion to the signal pulses. The gain of the YAG crystal rod 82 is determined by the efficiency of the crystal, the divergence of pump light and the signal pulses, and the overlap of the pump light and the signal pulses through the YAG crystal rod 82.

(37) In order to prevent optical feedback from the YAG crystal 82 into the amplifier fiber 62, both the output end of the amplifier fiber 62 and the YAG crystal rod 82 are coated with anti-reflection coatings. Alternatively, the amplifier fiber 62 and the YAG crystal rod can be fused together.

(38) It will be appreciated that a rod amplifier such as the YAG crystal rod 82 may also be used with any of the embodiments described above.

Seventh Embodiment

(39) FIG. 7 shows a high power short optical pulse source 90 according to a seventh embodiment of the invention. The optical pulse source 90 is based on the optical pulse source 40 of FIG. 2, with the following modifications. The same reference numbers are retained for corresponding features.

(40) In this example the high power short optical pulse source 90 further comprises nonlinear optical frequency conversion means 92, optically coupled to the output of the high power optical fiber amplifier means 20 and located within the optical head 18.

(41) The nonlinear optical frequency conversion means 92 comprises (optically coupled in series) a set of phase plates 94, a first nonlinear crystal 96, which in this example is a Type I LBO crystal, a second nonlinear crystal 98, which in this example is a Type I BBO crystal, and a harmonic separator 100.

(42) The output optical pulses from the high power optical fiber amplifier means 20, having a wavelength of 1064 nm and an average power of approximately 20 W, pass through the phase plates 94, to set the optimum state of polarisation for the nonlinear frequency conversion. The pulses are then launched, using conventional collimation and focusing optics, into the LBO crystal 96, to convert the wavelength of the pulses to their second harmonic of 532 nm, at an average power of 12 W.

(43) The frequency converted pulses output from the LBO crystal 96 are then launched into the BBO crystal 98, where the wavelength is converted to their fourth harmonic wavelength of 266 nm, at an average power of 1.2 W. The output from the BBO crystal 98 is then passed through the harmonic separator 100, which transmits only the desired wavelength, in this case 266 nm, as the output of the high power short optical pulse source 90 to the intended target.

(44) The LBO and BBO crystals 96, 98 are respectively mounted within ovens, controlled by a temperature controller 102. Electrical power for the ovens and the temperature controller 102, and to a cooling fan (not shown) for the high power optical fiber amplifier means 20 is delivered to the optical head 18 through a power cable 104.

(45) It will be appreciated that the nonlinear optical frequency conversion means 92 could alternatively use other combinations of crystals to generate 3.sup.rd, 4.sup.th or higher harmonics, for example periodically poled nonlinear crystals such as periodically poled lithium niobate (PPLN) or periodically poled potassium titanyl phosphate (PPKTP), or could use other combinations of conventional crystals such as LBO, BBO, BiB.sub.3O.sub.6 (BIBO), cesium lithium borate (CLBO), and potassium titanyl phosphate (KTP).

(46) For efficient frequency conversion the spectral bandwidth of the pulses must be maintained as narrow as possible. By transmitting the optical pulses through the greatest length of fiber before amplifying the pulses close to the nonlinear optical conversion means 92, the high power short optical pulse source 90 ensures that spectral distortion of the pulses is minimised and nonlinear optical conversion efficiency is thereby maximised.

(47) The high power short optical pulse source 90 can therefore deliver UV light directly to an intended target whilst remotely positioning the bulky elements of the source 90 away from the intended target.

(48) It will be appreciated that the nonlinear optical frequency conversion means 92 may be is included in any of the high power short optical pulse sources according to the previously described embodiments.

Eighth Embodiment

(49) A high power short optical pulse source 110 according to an eighth embodiment of the invention is shown in FIG. 8. The pulse source 110 of this embodiment is based on the pulse source 10 of the first embodiment, with the following modifications, and is operable to generate ultrashort (femtosecond) optical pulses. The same reference numbers are retained for corresponding features.

(50) The optical pulse source 110 further comprises a pulse compressor 112 provided within the optical head 18 and optically coupled to the output of the high power optical fiber amplifier means 20.

(51) In this example the pulse compressor takes the form of a transmission grating pair 112 having a grating separation of less than 10 cm. The pulse compressor 112 could alternatively comprise a fiber pulse compressor based on Photonic Crystal Fiber with anomalous dispersion at the signal wavelength.

(52) The master oscillator 12 generates optical pulses having a duration of approximately 10 ps at a pulse frequency of 10 MHz. The pulses are then amplified by the optical fiber preamplifier 14 to an optical power of approximately 50 mW. The signal pulses and 20 W 975 nm pump light from the pump light source 16 single are delivered over the signal delivery fiber 22 and the pump delivery fiber 24 respectively, each of which are 2 m long in this example, to large mode area, polarisation maintaining, single-mode Ytterbium doped amplifier fiber within the high power optical fiber amplifier means 20. The high power optical fiber amplifier means 20 amplifies the average pulse power to approximately 12 W, resulting in a pulse energy of greater than 10.

(53) During amplification within the high power optical fiber amplifier means, the pulses (which enter the high power optical fiber amplifier means with a duration of approximately 10 ps) becomes linearly chirped due to the combination of positive fiber dispersion and self-phase modulation.

(54) The amplified pulses are then passed through the transmission grating pair 112, which acts to compress the pulses to less than 500 femtoseconds duration, with an efficiency of greater than 80%, to produce pulses having an energy of approximately 10 and peak powers of 2 MW. The high power ultrashort pulses can then be delivered from the head 18 which can be positioned directly at the intended target without the need for complex beam steering optics.

(55) It will be appreciated that a pulse compressor 112 may be incorporated into any of the above described high power short optical pulse sources in order to generate ultrashort, femtosecond pulses.

Ninth Embodiment

(56) FIG. 9 shows a high power short optical pulse source 120 according to a ninth embodiment of the invention. The pulse source 120 is substantially the same as the pulse source 40 of FIG. 2, with the following modifications. The same reference numbers are retained for corresponding features.

(57) In this embodiment the pump light sources 16, 42 are provided within the first enclosure 28, at the first location, together with the pump sources 122, 124 for the preamplifier 14, and their associated electrical drive circuitry, cooling and heat dissipation elements (not shown). The high power optical fiber amplifier means 20 and the optical isolator 26 are provided within the optical head 18, at a second location, remote from the first location.

(58) The master oscillator 12 and the amplifier fiber and pump signal combiner of the optical fiber preamplifier 14, together with their associated isolators 30, 32, are provided within a second enclosure 126, at a third location intermediate the first and second locations, 126. The outputs of the preamplifier pump sources 122, 124 are delivered to the amplifier fiber of the optical fiber preamplifier 14 respectively via preamplifier pump delivery fibers 128, 130

(59) The master oscillator 12, preamplifier 14, and high power optical fiber amplifier means 20 can thus be enclosed in one or more separate enclosures positioned close to the application target area thus avoiding the need for beam steering optics.

(60) It will be appreciated that this arrangement can also be applied to any of the previously described embodiments, including those with nonlinear frequency conversion and pulse compression.

Tenth Embodiment

(61) FIG. 10 shows a high power short optical pulse source 140 according to a tenth embodiment of the invention. The pulse source 140 is substantially the same as the pulse source 10 of FIG. 1, with the following modifications. The same reference numbers are retained for corresponding features.

(62) The pulse source 140 further comprises a pulse picker 142 operable to reduce the optical pulse frequency. In this example the pulse picker 142 comprises an acousto-optic modulator driven at a synchronised gating frequency lower than the optical pulse frequency output from the master oscillator 12. The pulse picker 142 is provided after the preamplifier 14, within the first enclosure 28. The pulse picker 142 is located at a position within the pulse source 140 where the optical pulses have a low pulse energy. This is because the optical insertion loss into the acousto-optical modulator pulse picker 142 is high, so it is preferable for the pulses to be transmitted through the pulse picker 142 prior to amplifying them to their final high power. The provision of a pulse picker also enables higher pulse energies to be achieved for a given average output power.

(63) It will be appreciated that a pulse picker 142 may be provided within any of the above described pulse sources, and that where the pulse source comprises high power optical fiber amplifier means having two or more amplifier stages, the pulse picker 142 may be provided after any one of the amplifier stages between the master oscillator and the final high power amplifier stage.

Eleventh Embodiment

(64) A high power short optical pulse source 50 according to an eleventh embodiment of the invention is shown in FIG. 11. In this example, the optical head 18 is movably mounted on a translation stage 152 which is operable to provide translation of the optical head 18 in one or more of the x-, y- and z-axes. The controlled movement of the optical head 18 provided by the translation stage 152 enables accurate direction of the output high power short optical pulses onto a target or specimen under test. The optical head 18 may alternatively be mounted on a mechanical manipulator such as a robotic arm.

(65) It will be appreciated that the optical heads 18 of any of the previously described embodiments may be similarly movably mounted.

(66) Various modifications may be made to the described embodiments without departing from the scope of the invention.

(67) It will be appreciated that the master oscillator could alternatively comprise any optical source operable to generate short optical pulses, such as a solid-state modelocked laser, for example a Neodynium Vanadate laser, or a gain-switched laser diode. The amplifier fibers of the preamplifier and the high power optical fiber amplifier means may alternatively be doped with a different rare-earth dopant, such as Erbium, Thulium, or Neodynium.

(68) In most applications, the pump source's will be a multi-mode pump source operating at a suitable wavelength to pump the rare-earth-doped amplifier fiber (e.g. 808 nm, 915 nm or 975 nm). For example, a single fiber-pigtailed laser diode, diode bar or multi-emitter pump source such as the PUMA source commercially supplied by Fianium Ltd, which delivers between 20 and 40 W within a single, 105 μm core, 0.22 NA pump delivery fiber.

(69) Where multi-mode pump delivery fibers are used, the pump signal combiner will typically include a conventional tapered bundle, such as those commercially available from OFS or Sifam Fiber Optics. Alternatively, the pump signal combiner may be a tapered coupler, a side-pumping fiber configuration, or a distributed pump coupling fiber configuration such as that proposed in U.S. Pat. No. 6,826,335. In the multi-mode pumping regime, the amplifier fiber typically comprises a double-clad fiber structure where the signal pulses are guided within a signal core and the pump light is guided within the fiber cladding.

(70) Alternatively, the pump source/s can be a single mode pump source such as a fiber-pigtailed laser diode or a fiber laser. The fiber laser can be a Raman fiber laser or could be a cladding pumped fiber laser generating cw or pulsed radiation at the wavelength required to pump the rare-earth-doped amplifier fiber. In the single-mode pump regime, the signal pulses and pump light from the one or more pump sources are combined using conventional wavelength-division multiplexing (WDM) components such as fused WDM couplers or thin-film WDM couplers.

(71) In the single-mode pump regime, the amplifier fiber has a conventional single clad geometry, where both the signal pulses and the pump light propagate within the core of the amplifier fiber. This has benefits in increasing the pump absorption and thus reducing amplifier length and hence nonlinearity.

(72) The optical head of any of the high power short optical pulse sources described may also include a heat sink and integrated fan within the high power optical fiber amplifier means, to provide forced-air cooling of the amplifier fiber. Optical monitoring means may also be provided within the optical head, with both electrical and optical communication to the first enclosure to monitor feedback and power levels, enabling interlocks and automatic fault detection.

(73) The described embodiments provide various advantages, as follows.

(74) The high power short optical pulse sources enable a short-length fiber amplifier system and post processing optics to be remotely located from the bulky, noisy heat producing elements of the pulse source, and thereby enable optical head to be very compact. The optical head can therefore be positioned close to the application space, taking up minimal space and avoiding the need for complex and expensive beam steering optics.

(75) The high power short optical pulse sources enable the delivery of ultrashort (femtosecond) optical pulses with high energy and narrow spectral bandwidth, without the use of complex chirped pulse amplification (CPA) methods. In short optical pulse amplifier systems which don't incorporate CPA methods, it is essential that all fiber lengths which deliver or amplify pulses to high peak powers, are kept as short as physically possible; the structure and configuration of the high power short optical pulse sources ensures that this is achieved.

(76) The high power short optical pulse sources incorporating a pulse compressor eliminate the need for CPA in fiber-delivered laser sources. Providing a high power ultrashort pulse source which is more robust, lower cost and easier to maintain and assemble than known ultrashort pulse sources using CPA pulse compression.

(77) The high power short optical pulse sources further enable the delivery of UV optical pulses, efficiently generated within the optical head.

(78) The high power short optical pulse sources also provide the advantage of beam delivery directly to the target without the use of complex beam steering optics. The high power short optical pulse sources therefore offer improved mechanical robustness, reduced cost and weight, and ease of maintenance over known high power optical pulse sources.

(79) The high power short optical pulse sources also allow for scalability in power because they can incorporate multiple pump light sources in the remote pump enclosure and the pump delivered over long lengths with very low loss to the amplifier system which is remotely located whether within or close to the output optical head.