High gain semiconductor laser amplifier package

Abstract: A laser amplifier package comprises two laser amplifiers (1 and 2) coupled via two lenses (5 and 6) and an optical isolator 7. The first and second laser amplifiers may both be run as amplifiers to give an overall improved gain and reduced residual facet reflectivity. The first amplifier may be run as a pulse source as part of an LEC or gain switched DFB laser assembly. Pulses generated to be, or compressed to, near transform limited enable the output from the package to be optical solitons. The second laser amplifier may be switched to provide modulation.

Description: This invention relates to high gain semiconductor laser amplifiers, and more particularly to the combination of two such laser amplifiers. Travelling wave semiconductor laser amplifiers have been proposed for a number of optical communications applications. They have been used, for example, as repeaters, preamplifiers, power boosters, switches and filters. Many of these applications rely on high gain to achieve optimum performance and a critical factor in achieving high saturated output power is minimising residual facet reflectivity. Up to the present time, only limited gains of around 20dB have been demonstrated, and it has not yet proved possible to improve on this due to the difficulty in reducing residual facet reflectivity. The present invention is directed in a first aspect towards tackling both of these problems, and producing a laser amplifier package having higher gain than previously achieved. When a pulsed optical signal is propagated along an optical fibre link there is usually pulse spreading due to dispersion and thus it is necessary to regenerate the signal at repeaters spaced along the link. However it is possible to propagate optical pulses along an optical fibre without significant dispersion, if the pulses are optical solitons, and in that case very long transmission distances without regeneration becomes possible. Optical solitons are relatively high peak power (eg 100mW), short duration (eg 3ps) pulses of smooth profile. Generation of optical solitons is at present achieved utilising cryogenic lasers and the equipment is large and expensive. See, for example, US patent 4,635,263, in which a colour centre laser is used as a pulse source. The present invention in a second aspect is directed towards the production of optical solitons utilising semiconductor lase amplifiers. Another aspect of the invention is directed towards modulation techniques for use in the first and second aspects of the invention. The present invention provides a laser amplifier package comprising first and second semiconductor laser amplifiers and an optical isolator, the output of the first laser amplifier being applied to the second laser amplifier via a collimated beam section constituted by two lenses, and the optical isolator being positioned between the two lenses, wherein the optical isolator has an isolation factor such that the round trip signal loss for a signal which passes in the forward direction through the second laser amplifier, is reflected by the remote facet thereof, is passed through the second laser amplifier in the reverse direction, and is passed through the optical isolator, is between 8dB and 10dB. The invention also provides a method of amplifying an optical pulse the method comprising supplying an input optical pulse via an optical fibre to a sealed package comprising first and second semiconductor laser amplifiers arranged in series, amplifying the input optical pulse in the first laser amplifier, feeding the resultant amplified optical pulse from the output of the first laser amplifier via a cultimated section constituted by two lenses into the second laser amplifier, further amplifying the optical pulse in the second laser amplifier, and outputting via an optical fibre the resultant further amplified optical pulse, wherein an optical isolator being positioned between the two lenses and is provided between the first laser amplifier and the second laser amplifier, the optical isolator having an isolation factor which is such that the round trip signal loss for a signal which passes in the forward direction through the second laser amplifier, is reflected by the remote facet thereof, is passed through the second laser amplifier in the reverse direction, and is passed through the optical isolator, is between 8 and 10dB. The invention also provides a soliton generator comprising in a package a semiconductor laser, a semiconductor laser amplifier, and an optical isolator, the output of the laser being applied to the input of the laser amplifier by a collimated beam section constituted by two lenses, and the optical isolator being positioned between the two lenses, wherein the laser is driven as a pulse source, and part of an LEC laser assembly which is actively mode locked so that its output pulses are near transform limited, or as a gain switched DFB laser producing nontransform limited pulses in which case the laser amplifier is a non-linear amplifier which is such as to compress the pulses to near transform limited pulses, and wherein the pulses are fed to the laser amplifier where they are amplified to form optical solitons transmitted through an optical fibre. The first amplifier may be driven as a pulse source as part of a long-external cavity (LEC) or gain switched DFB laser assembly. The pulses may be made near transform limited (see chapter 9 of 'Lasers', by Anthony E. Siegman, Oxford University Press, 1986, in particular pages 334-335) at generation or by compression and the amplification sufficient to produce optical solitons. A more detailed description of a specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: - Figure 1 shows a schematic diagram of an embodiment of the invention; Figure 2 shows a schematic diagram of an experimental apparatus used for testing the embodiment of the invention shown in Figure 1; and Figure 3 is a modification of the package of Figure 1 suitable for generating solitons. Very surprisingly, we have discovered that when two laser amplifiers are aligned in series, as little as 20dB of optical isolation may be sufficient to prevent the backward amplified wave (signal and spontaneous) from the second amplifier affecting the first. Previously it was believed that, if one were to put two laser amplifiers in series, in order to prevent interaction one would need to reduce the level of back spontaneous emission (which would typically be about 1mW) to about 10 percent of the signal level. To do this would typically require 40dB of isolation. What we have discovered is that the critical factor is not back-spontaneous emission, but is the returned signal level. Because of the influence of the amount of gain provided by the second device on the level of reflected signal, the loss which needs to be provided by the isolator is related to the gain of the second device. A signal reflected from the remote facet of the second device is typically 30dB down, so with a 20dB gain device, a 20dB isolator gives an overall 10dB loss (ie in the second device the forward signal gain is 20dB, there is a 30dB loss on reflection, then a 20dB gain for the reflected signal, which is a total of 10dB gain, then 20dB loss through the isolator gives a 10dB overall loss). We have found such a 10dB overall loss completely satisfactory, and indeed one could accept as little as 8dB overall loss. Conversely, greater overall loss could be accepted, but this would normally mean that one was either using an over-complex isolator or one was not driving the gain of the second device as hard as one could be. Typically therefore if one had a 30dB isolator one would push the gain of the second device to 30dB, if possible. As a result of our discovery, we realised that it would be possible to mount a small, and hence only modest performance, optical isolator between two laser amplifiers, and to fit the whole assembly into a compact device package. This would facilitate alignment and the control of reflections, hence making such a device cheap and readily manageable. The system, illustrated schematically in Figure 1, is based on two 500µm long buried heterostructure semiconductor laser amplifiers, 1 and 2, each with a gain peak around 1.5µm and with residual facet reflectivities of 2x10⁻³. Amplifiers 1 and 2 are arranged in series. Input and output coupling to the system is achieved via anti-reflection coated, tapered lensed ended single mode fibres 3 and 4 with a lens radius of approximately 10µm, and a coupling loss of 6dB composed of 3dB at the input 3 and 3dB at the output 4. The amplifiers are coupled together, via a collimated beam section, with a loss of 6dB, using two anti-reflection coated, Gadolinium Garnet (GGG) spheres 5 and 6, chosen in this embodiment to have a diameter of 1mm, which are each placed preferably at a distance of 5 to 10µm from one of the amplifier facets. If one or both of the spheres are misaligned the coupling loss will be increased. A simple miniature optical isolator 7 is placed in the collimated beam section. The optical isolator 7 comprises two dielectric polarising beamsplitter cubes 8 and 9, a YIG Faraday rotator 10 placed in a samarium cobalt magnet 11, and a half wave plate 12 which corrects the net single pass polarisation rotation. The dimensions of these components in this particular embodiment of the invention are preferably chosen to be 3mm per side of the beamsplitter cubes 8 and 9, a 3mm diameter YIG Faraday rotator 10, and a 1.5mm diameter magnet 11. In this particular embodiment the isolator has an insertion loss of 2dB and an isolation of 20dB. This degree of isolation is, surprisingly, sufficient to prevent the backward amplified wave (signal and spontaneous) from the second amplifier affecting the first amplifier. Thus the isolator prevents the system from oscillating, and eliminates crosstalk effects between the signal and amplified reflections. The two amplifiers 1 and 2 have supports which rest on peltier coolers 13 and 14. The whole assembly is mounted inside an hermetic package 3cm long and less than 2cm wide. Tests were carried out on the embodiment of Figure 1 using apparatus which is schematically illustrated in Figure 2. The peak gain, gain saturation and gain ripple of the system were measured using a distributed feedback (DFB) laser, emitting at 1.51µm, as a transmitter 15. This was connected to the input of the system by polarisation controllers 16 and 17, an optical isolator 18, an optical attenuator and a directional coupler 20 used for monitoring. The receiver 21 connected to the output of the amplifier system or package 22 was a PIN diode connected, via a 40dB amplifier, to an oscilloscope. The double amplifier package had a "device gain" (input facet to output facet) of 32dB, or a net gain of 26dB. The DFB laser was tuned through two residual Fabry-Perot peaks of the amplifier by adjusting the DFB laser operating temperature and the variation in the output power of the package was measured. The input power to the amplifier package was maintained at -30dBm for all temperatures, and thus the gain ripple for 32dB gain was only 3dB. The results were obtained with both amplifiers biased at the normal operating current of the devices (40mA), where a device gain of 21-22dB per amplifier would be expected with a gain ripple of 3dB in each device. Since the loss in the package between the amplifiers was 8dB it was concluded that the gain saturation of the second amplifier due to the sponteneous emission of the first amplifier was 4dB. When the bias on the second amplifier was increased to 55mA, to overcome the gain saturation, and the amplifiers were retuned (to realign the residual Fabry-Perot modes) the gain was increased by 3dB. The total gain ripple of the package at the higher gain of 35dB was 6dB. The effective reflectivity, derived using a Fabry-Perot analysis, was found to be 2x10⁻⁴. This reduction of facet reflectivity is a result of using an optical isolator between the two amplifiers. The backward amplified signal from the package was measured to be 20dB smaller than the forward amplified signal. This is consistent with the effective reflectivity and is a great advantage if the amplifier is to be used in a transmission system requiring more than one repeater. The tests also determined the gain saturation characteristic of the package. 3dB gain compression occurs for an input power of -27dBm. This is very similar to the figure for a single amplifier of this type with a gain of 20dB and would be improved further by using better individual amplifiers than those detailed above. The 3dB gain compression for a single amplifier with a gain of 35dB occurs for an input power of -40dBm. To use the package as a receiver preamplifier the output fibre of the package was then removed and replaced by a lensed photodiode (which was terminated into 50ohms). The coupling loss to the photodiode was 2dB so the net gain of the package was now 30dB. The sensitivity of this receiver was -35dBm at 565 Mbit/s for amplifier gains in excess of 20dB. This is 3dB worse than the sensitivity of a similar receiver constructed using a single amplifier and shows that there is no excess noise penalty associated with the use of two amplifiers, since a 3dB penalty is predicted from the increase in Δ fl associated with the reduction in effective reflectivity. Insertion of a 3nm bandpass filter between the second amplifier and the photodiode increased the sensitivity to -39.5dBm. Insertion of a similar filter between the amplifiers (in addition to the isolator) would lead to a further increase in the sensitivity. This particular package was not constructed with such a filter since this would restrict the operating range of a linear repeater and for this application the broadband spontaneous emission noise is less significant. The increased gain available in this package will enable construction of receivers whose sensitivity is limited by amplifier noise at any bit rate, within the gain bandwidth of the amplifier. The principal problem with this approach to making a high gain amplifier package is the high polarisation sensitivity of 30dB due to the use of an optical isolator. Polarisation insensitive amplifiers are now available, however, and by combining them with a polarisation insensitive optical isolator a further improvement to the package should be possible. The package as described in this embodiment of the invention is a two amplifier package with a gain of 30dB, an effective reflectivity of 2x10⁻⁴, a gain ripple of 6dB, and no noise penalty. One significant advantage of the combination described is that the isolation required is markedly less than that which would have been predicted; only 20dB being required when it would have been expected to need 40dB. This package is ideal for use as a linear repeater since it would reduce the number of repeaters required in a long distance transmission system. With the simple modification described above the package can also be used very successfully as a receiver preamplifier, particularly at high bit rates. A modification to the package is illustrated schematically in Figure 3. In this embodiment a first laser amplifier is a mode locked resonant device in a laser assembly 23 with a controlled grating 24 defining an external cavity. Pulses generated by this mode-locked laser arrangement can be made near transform limited to give the narrowest spectrum possible in the time domain, which occurs when ΔtΔν≃.315 where Δt is the full width half maximum in time and Δν is the full width half maximum laser linewidth. The pulses that are generated by the mode locked laser are input to the second laser amplifier and amplified to solitons. For standard single mode fibre the following general values for pulse length, repetition rate and mean output power are required to provide solitons. Since it is now possible to have a semiconductor laser amplifier with a saturated output power of 10mW at the output facet it is possible, utilising a double laser amplifier package according to the invention, to generate for example 20ps solitons at a rate of 5GHz assuming that the coupling loss into the fibre is less than 3dB. It is expected that saturated output powers of 20mW will soon be available and ultimately about 100mW maximum is possible which would enable 5ps solitons at 20GHz which would be of significant practical use. A more detailed description of Figure 3 now follows. The laser chip for the mode locked laser assembly 23 was a 500µm long buried heterostructure laser with a multilayer anti-reflection coating on one facet. The residual facet reflectivity was 4x10⁻⁴ and the laser had a large signal modulation bandwidth of 1GHz. The output from the laser chip was coupled into a 8cm cavity via a 1mm diameter sphere lens with an output beam diameter of 300µm. The grating 4 was a 600 line/mm ruled grating blazed for 1.6µm and gold coated. The feedback bandwidth from the grating was 300GHz (the resolution of the grating for a 300µm beam) and the dispersion due to the grating angle was 0.5ps. The laser was actively mode locked by superimposing a sine wave (in the frequency range 500MHz to 20GHz) on a 25mA dc bias. When the mode-locking was optimised the output of this laser consisted of a train of near transform limited 1.5ps pulses (with a time bandwidth product of 0.35) at a repetition frequency of 1.6GHz which could be tuned over 30nm. The pulses could be reproduced at cavity harmonic frequencies up to 8GHz by harmonic mode locking. Near transform limited pulses with widths of less than 20ps could be obtained over a bandwidth of 110nm. The output pulses were measured using a streak camera with time resolution of 2ps and had a minimum measured width of 2.5ps. The pulse spectra were measured using a monochromator and had a maximum width of 1.9nm. The laser was tuned to emit at 1.52µm and the pulses were transmitted via an optical isolator (60dB) and 10m of fibre (with a dispersion of 0.2ps) to an optical amplifier. In an alternative construction, illustrated in Figure 3, the output of the isolator was fed directly, that is without the aid of a fibre, into an optical amplifier. The amplifier was a 500µm long buried heterostructure chip with multilayer anti-reflection coatings on both facets. The residual facet reflectivity was 8x10⁻⁴ and the gain ripple at the operating current of 100mA was 3dB with a device gain of 25dB. The material dispersion of the chip (derived from the emission spectrum) was 0.05ps. Output coupling was achieved using ground tapered lens ended fibres and, where a similar fibre was used to couple into the amplifier, the total coupling loss was 9dB. The maximum mean input power to the amplifier was -16dBm at the facet and this resulted in a 3dB reduction in gain from the unsaturated gain of 25dB. The output of the amplifier was measured on a streak camera and a monochromator. The results, for an input power of -16dBm, showed that the pulse has suffered no spectral shift or distortion and only a slight broadening consistent with the dispersion of the measurement system was observed even though the amplifier gain is saturated. The gain and gain saturation were found to be independent of the pulse repetition frequency at all available frequencies up to 8GHz. The gain recovery time of the amplifier was estimated to be 500ps by measuring the amplitude compression of secondary pulses (which could be obtained from the mode locked laser when the operating conditions were not optimised) after amplification. The mean input power required for 3dB gain compression at the gain peak (1.5µm) was -19dBm (at the facet) and the mean input power required for 3dB gain compression at 1.52µm was -16dBm. The maximum average output power of the amplifier was 2mW. This could have been increased by increasing the input power but pulse distortion due to gain saturation would occur. However in order to transmit 1.5ps pulses over an appreciable distance fibre dispersion must be compensated for, possibly by using soliton effects, for which the output power would need to be 26mW with a 1.6GHz repetition frequency. This demonstrates that the semiconductor laser amplifier has sufficient bandwidth to be useful in ay conceivable linear optical communication system but development of high output power amplifiers is required in order to maximise its potential for use in non-linear systems. An alternative to the mode locked laser arrangement shown in Figure 3 would be to use a gain switched DFB laser. Such a source would not provide transform limited pulses, but by utilising a non-linear amplifier for the second laser amplifier, pulse compression to transform limited pulses can be achieved, along with amplification, in the second amplifier. Pulse compression may be achieved in two ways, one way is to chop the front end of the pulse by having for example a two element amplifier with a saturable absorber element that absorbs the front part of the pulse and transmits the rest of the pulse after it reaches saturation. Alternatively the back end of the pulse may be chopped off by using gain saturation in which the carriers are depleted by the first part of the pulse so that the latter part of the pulse is not transmitted. A gain switched DFB laser is less sensitive to feedback than an LEC laser and the isolation between the two laser amplifiers may therefore be reduced possibly to of the order of 40dB. Having achieved a stream of soliton pulses it is, in order to utilise them for data transmission, necessary to modulate the pulses with data. This is also the case for non-soliton pulse streams that may be generated using a package similar to that in Figures 1 or 3. In the present invention it is envisaged that modulation can be achieved by modulating the bias on the second amplifier. If the bias is turned on and off at a rate of up to 2GHz then pulses will be correspondingly turned off and on. However if a higher bias switching rate is used, for example 10GHz, then the amplifier does not fully turn off and the result is a ripple on the gain pulse, a typical loss being 3dB for the 'off' pulses. The modulated pulses may then be further processed to eliminate the lower intensity pulses. If the laser amplifiers are set to produce solitons then the 3dB loss for the 'off' pulses will reduce their power below soliton level and the pulse train may be filtered through a soliton loop mirror switch so that only soliton pulses and zero intervals (corresponding to the switched out lower intensity pulses) are launched into the fibre. In both the first and the second embodiments, modulation at the second laser amplifier may also be used to provide a supervisory or response channel, for example incoming data pulses to the first laser amplifier of the package of Figure 1 may be read (eg by monitoring the voltage over the device) and a modulation imposed at the second amplifier indicative of the fact that the data has been read.

Claims: A laser amplifier package comprising first and second semiconductor laser amplifiers and an optical isolator, the output of the first laser amplifier being applied to the second laser amplifier via a collimated beam section constituted by two lenses, and the optical isolator being positioned between the two lenses, wherein the optical isolator has an isolation factor such that the round trip signal loss for a signal which passes in the forward direction through the second laser amplifier, is reflected by the remote facet thereof, is passed through the second laser amplifier in the reverse direction, and is passed through the optical isolator, is between 8dB and 10dB. A laser amplifier package as claimed in claim 1, wherein the isolation factor of the optical isolator is no more than 30dB. A laser amplifier package as claimed in claim 1 or claim 2, wherein the isolator is constituted by a Faraday rotator and a half wave plate positioned between two dielectric polarising beamsplitter cubes. A method of amplifying an optical pulse the method comprising supplying an input optical pulse via an optical fibre to a sealed package comprising first and second semiconductor laser amplifiers arranged in series, amplifying the input optical pulse in the first laser amplifier, feeding the resultant amplified optical pulse from the output of the first laser amplifier via a collimated beam section constituted by two lenses into the second laser amplifier, further amplifying the optical pulse in the second laser amplifier, and outputting via an optical fibre the resultant further amplified optical pulse, wherein an optical isolator being positioned between the two lenses and is provided between the first laser amplifier and the second laser amplifier, the optical isolator having an isolation factor which is such that the round trip signal loss for a signal which passes in the forward direction through the second laser amplifier, is reflected by the remote facet thereof, is passed through the second laser amplifier in the reverse direction, and is passed through the optical isolator, is between 8 and 10dB. A soliton generator comprising in a package a semiconductor laser, a semiconductor laser amplifier, and an optical isolator, the output of the laser being applied to the input of the laser amplifier by a collimated beam section constituted by two lenses, and the optical isolator being positioned between the two lenses, wherein the laser is driven as a pulse source, and part of an LEC laser assembly which is actively mode locked so that its output pulses are near transform limited, or as a gain switched DFB laser producing non-transform limited pulses in which case the laser amplifier is a non-linear amplifier which is such as to compress the pulses to near transform limited pulses, and wherein the pulses are fed to the laser amplifier where they are amplified to form optical solitons transmitted through an optical fibre. A soliton generator as claimed in claim 5, further comprising a modulator for modulating the bias on the laser amplifier. A soliton generator as claimed in claim 6, wherein the modulator is such as to alternate the bias on the laser amplifier between on and off. A soliton generator as claimed in claim 7, wherein the modulator is such that the laser amplifier is switched at a rate such that it is not fully turned off. A soliton generator as claimed in any one of claims 6 to 8, further comprising means for filtering lower intensity pulses output when the modulator is such that the bias on the laser amplifier is switched off.

Inventors: MARSHALL IAN WILLIAM, MARSHALL, IAN WILLIAM
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: H01S3
IPC codes: G02F1, H01S3, H01S5, G02B6