Optical amplifier

Abstract: A laser amplifier operable such that at the commencement of each output pulse the carrier level has receovered from depletion by the previous pulse. Under these conditions output power commensurate with a much high carier density than is usually available is possible. The mode of operation is especially useful as a way of providing high power pulses, for example for use in optical time demain reflectometry. Using a travelling wave amplifier in conjunction with return to zero formate data pulses is also disclosed.

Description: This invention relates to optical amplifiers. Laser amplifiers are of great interest as components in optical communication systems. Unfortunately, the devices suffer from output power saturation at relatively low power, that is in the range of 1 to 10 mW mean power. There are also various applications for which it is desirable to have high power optical pulses, but at present the equipment necessary to provide such pulses is both expensive and bulky. The present invention is directed towards obtaining relatively high power optical pulses from laser amplifiers, and in preferred embodiments towards providing portable high power optical sources. The present invention provides a method of generating output pulses using a semiconductor laser amplifier of the type having a semiconductor medium exhibiting carrier density depletion, the method comprising the steps of: (a) applying a bias current to the laser amplifier less than the bias current required to induce lasing in the amplifier, the bias current establishing an internal carrier density within the amplifier; (b) exciting the amplifier with optical pulses of sufficiently high pulse energy that the excitation substantially completely depletes the established internal carrier density; (c) providing the excitation optical pulses with a time interval between pulses in excess of twice the time required for the amplifier internal carrier density to substantially recover from said depletion, wherein the excitation pulses have a duration of substantially less than said recovery time, and (d) taking the output of the amplifier in the period during which the established internal carrier density is being depleted. The invention also provides an optical pulse source comprising a semiconductor laser amplifier arranged in operation to optical pulses generate output having a peak power of at least 100 mW in response to optical excitation, the amplifier exhibiting carrier concentration depletion in response to said excitation, and drive means optically coupled to the amplifier arranged in operation to generate optical excitation pulses and to apply a series of such excitation pulses to the amplifier, the drive means including means to provide a time interval between pulses in excess of twice the time required for the amplifier internal carrier density to substantially recover from said depletion, wherein the drive means is arranged to produce excitation pulses of sufficiently high pulse energy that the excitation substantially completely depletes the established internal carrier density, and wherein the drive means is such as to control the durations of said excitation pulses to be substantially less than the time the amplifier takes to recover from said depletion, thereby in use to generate said optical output pulses in the period during which the established internal carrier density is being depleted. Preferably the amplifier is a travelling wave amplifier. The term 'optical' as used herein includes those portions of the electromagnetic spectrum termed 'infra red', visible and 'ultra violet'. The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 shows carrier density versus time in a semiconductor laser amplifier; Figure 2 shows a gain characteristic for a travelling wave amplifier; Figure 3 is a schematic diagram of an embodiment of the invention, and Figure 4 shows input versus output power for a laser amplifier operated in accordance with the invention. Semiconductor laser amplifiers have heretofore been regarded as limited in output power to the order of 1 to 10 mW, and this limitation has been accepted in known applications of the device. However, the present invention is based upon a consideration of the instantaneous carrier concentration available and operation of the device so as to increase or maximise the carrier concentration at the moment of power output demand. In a semiconductor laser the optical power output is limited by the internal carrier density as it is these carriers that produce the output power by electron-hole recombination in response to the input light signal. The carrier density is a function of the applied bias current, but there is a limit to the current that can be applied because for wideband operation the laser amplifier needs to be operated short of the lasing threshold, typically the bias being 70% of the lasing bias threshold. In the absence of an input light signal the carrier density is static at a level determined by the bias, but as soon as a signal is input that stimulates recombination there is a resultant drop in the carrier concentration. The bias current regenerates carriers but the effect is not instantaneous so that under conditions of sustained input signal (which may be continuous or rapidly repeating pulses such as a run of binary ones) the carrier level remains depleted below the starting level, and steady state operation can only by sustained at a power level based on the carrier concentration in the depleted state. When input signal light is removed the carrier concentration builds up to the original starting level. The time taken for the original starting level to be regained is in general equal to the mean carrier recombination time or carrier lifetime (although optical pumping or bias modulating may vary the parameters). The expression 'recovery time' is used herein to define the time taken for the carrier concentration to be restored to the undepleted level determined by the prevailing bias voltage when there is no input signal ie. the carrier lifetime. Typically the recovery time (carrier lifetime) may be of the order of 2ns. Figure 1 shows schematically carrier density versus time in a semiconductor laser amplifier for various excitation conditions. In Figure la there is first zero input signal, followed by a series of pulses, then an absence of signal followed by a single long pulse. This sequence could easily correspond to a data signal of binary zeros and ones with the absence of signal representing one or more zeros and the single long pulse being a run of consecutive ones. Under these conditions it can be seen that the carrier density rarely returns to the undepleted level 1; in general the carrier concentration is at a much lower level 2 and it is this lower level that determines the output power. It is not possible to utilise the occasional availability of higher carrier densities because these appear unpredictably (for example only after a sequence of zeros) and cannot be sustained. These are the conditions under which laser amplifiers have been used and illustrates how the accepted limitation arises. In contrast, in Figure 1b narrow pulses spaced apart by a distance sufficient to enable substantial recovery of the original carrier density means that the power available for each pulse is based upon the higher carrier density. This is the principle mode of operation according to the invention. As an intermediate step between conventional operation and that shown in Figure 1b, incomplete carrier density recovery, but still greater than that permitted by conventional operation, may be provided as shown in Figure 1c. However, according to the invention there should be substantially complete recovery. In this instance intermediate pulse powers are achieved. Use of return to zero data pulses and a narrow pulse width enable much higher powers to be achieved than previously, as seen by comparing Figures 1a and 1c. In order to achieve high power output from a laser amplifier that is achieving output power commensurate with a carrier density substantially greater than that which leads to output power saturation under conventional operation, requires observation of the following criteria: 1. The input pulse should have a pulse width less than the carrier lifetime (the recovery time of the amplifier), preferably substantially less. 2. The interval between pulses should permit substantial recovery to take place, and according to the invention this interval should be at least twice the time required for the internal carrier density to substantially recover from depletion. 3. If a data pattern is to be amplified the input should be in return to zero format to avoid runs of ones resulting in sustained depletion of carrier density. It should be noted that when operating in its conventional mode the carrier density in a laser amplifier remains substantially constant from operational considerations, that is the output is taken with the carrier density depleted to the lower level. However, the mode of operation according to the invention takes the output during a period of considerable change in carrier density, essentially during the drop from the maximum to minimum carrier densities, and this change in carrier concentration causes, in presently available materials, a change in the refractive index of the device medium, which results in wavelength shifts in the amplifier spectrum. Figure 2 shows (in full lines) the characteristics of amplifier gain versus wavelength for a Fabry Perot laser amplifier from which it can be seen that wavelength shift can have a dramatic effect on amplifier gain. The dotted line in Figure 2 shows the characteristic for a travelling wave or near travelling laser amplifier in which the gain fluctuation with wavelength is much reduced. For most practical purposes a gain ripple amplitude of about 3dB at the mean operating wavelength is tolerable (although in some instances 6dB or more may be acceptable), and so a fourth criterion of operation with carrier density sensitive material in the high power amplification mode of the invention is the use of a travelling wave or near travelling wave amplifier, for example by provision of antireflection coatings on one or more of the laser facets. In this specification 'travelling wave amplifier' is used to denote amplifiers of the type in which this gain ripple is eliminated or suppressed and includes amplifiers of the type termed 'near travelling wave amplifiers'. Within these constraints the pulse output can be made of higher peak power and shorter duration or of longer duration with lower power. If P p is the peak output power, P s the mean output saturation power, T r the repetition time and T the pulse width it has been found that the following approximate relationship holds approximately for non extreme values. P p = (T r /T) P s A practical embodiment utilising the above described mode of operation of a laser amplifier is shown schematically in Figure 3. A primary optical pulse source 3 has its output input to a pulse processor 4 and the pulse processor output is input to the laser amplifier 5. The pulse 3 source and pulse processor may be any components that provide pulses of suitable length and repetition frequency. For example the pulse source may be another laser that is mode locked, from which a stream of pulses of relatively high repetition rate is obtained with the pulse processor blocking most of the pulses so that only every n th pulse is selected and reaches the laser amplifier. Alternatively a source such as a constricted mesa laser may be electrically pulsed to provide pulses of suitable repetition rate, but these are too long in pulse length and so the pulse processor is then a pulse compressor. Another method of providing a suitable pulse source is to gain switch a laser source; the output of this process will usually require both pulse blocking or selection and pulse compression processing. Other possible sources are gas lasers with optical switches, solid state lasers such as iodine lasers and fibre lasers which may be mode locked or Q switched. A suitable laser amplifier 5, for example is a 1.5 microns DCPBH laser of 500 microns cavity length with antireflection coatings on both facets. The coatings may be single layer refractory oxide coatings to provide low facet reflectivities, for example of the order of 0.08% or less. With a pulse source and processor providing a pulse repetition T r of 10 ns a device with a peak saturation power P s of 1 mW achieved a peak pulse power of 250 mW for a pulse width of 40ps; a device with a peak saturation power of 10 mW yielded, for the same pulse period, 2.5W. As the pulse repetition time T r increases, the peak power increases until limited by other factors such as laser facet damage. Increasing the pulse width causes decay of the trailing part of the pulse when power cannot be sustained for the full pulse, this effect becoming evident at pulse widths of the order of 1-2ns. Figure 4 shows peak input power versus peak output power for the 1.5 microns DCPBH laser biased at 70% of the lasing threshold. In general it is preferred to operate travelling wave laser amplifiers at about 70% of the lasing threshold; at higher biases the ripples in the amplification tend to increase (eg 6dB at 95% of lasing bias compared to 3dB at 70%) and the bandwidth of the amplifier decreases. For data applications in which a return to zero format is used for the data a non-standard format is preferably adopted in which the pulse representing binary one occupies only a small proportion of the bit period. For recovery times (carrier lifetimes) of 1ns data rates of 1Gbit/sec are possible, the pulse format providing greater receiver sensitivity and hence system performance. It will be realised that when using a portable primary pulse source such as a semiconductor laser, the laser amplifier operated according to the invention provides a portable and comparatively inexpensive high power pulse source. A particularly important use of such a source is in optical time domain reflectometry (OTDR) in which a pulse of light is launched into a fibre that is to be surveyed for faults and any light returning due to reflection at faults is analysed in order to locate the fault. In this technique the length of fibre that can be surveyed is determined by the sensitivity of the detector and the energy of the pulse that is launched. For high resolution measurements, that is to survey accurately to within centimetres, pulse widths of the order of 100ps (more generally the range may be 10 to 200ps) are required. The present invention provides a possible increase of 20dB in input energy thus enabling either use of less sensitive detectors or survey of a greater length of fibre. For use in the field the portability of the equipment is of significant value. There are various other uses for a high power optical pulse source, especially in non-destructive testing techniques such as medical diagnostics. One technique currently under development is the monitoring of the concentration of oxygen in the brains of premature babies by high power pulses at infra-red wavelengths. At 1.3 or 1.5 microns the absorbtion of light by tissue (principally water) is low but absorbtion by other materials (like oxygen molecules) is high, so tests can be made through entire organs rather than needing to take a section. However in order to have enough power to provide measurements without causing burning, the source must be pulsed at high speed. Currently the available sources (dye lasers) are not portable. However, a source based on this invention is easily portable, enabling bedside installation and operation and is capable of providing comparable power. A particularly preferred pulse format is for the pulse to occupy at most one tenth of the pulse interval and for the pulse interval to be of the order of at least three times the recovery time (carrier lifetime).

Claims: A method of generating output pulses using a semiconductor laser amplifier of the type having a semiconductor medium exhibiting carrier density depletion, the method comprising the steps of: (a) applying a bias current to the laser amplifier less than the bias current required to induce lasing in the amplifier, the bias current establishing an internal carrier density within the amplifier; (b) exciting the amplifier with optical pulses of sufficiently high pulse energy that the excitation substantially completely depletes the established internal carrier density; (c) providing the excitation optical pulses with a time interval between pulses in excess of twice the time required for the amplifier internal carrier density to substantially recover from said depletion; wherein the excitation pulses have a duration of substantially less than said recovery time, and (d) taking the output of the amplifier in the period during which the established internal carrier density is being depleted. A method as claimed in claim 1, further comprising the step of controlling the repetition rate and duration of the optical pulses so as to maximise the peak output power of said amplifier. A method as claimed in claim 1 or claim 2, further comprising the step of generating the excitation pulses in response to a data encoding scheme which converts input data signals to return-to-zero format, the encoding scheme being such as to avoid runs of successive data pulses representing binary "1" values. An optical pulse source comprising a semiconductor laser amplifier arranged in operation to generate output optical pulses having a peak power of at least 100 mW in response to optical excitation, the amplifier exhibiting carrier concentration depletion in response to said excitation, and drive means optically coupled to the amplifier arranged in operation to generate optical excitation pulses and to apply a series of such excitation pulses to the amplifier, the drive means including means to provide a time interval between pulses in excess of twice the time required for the amplifier internal carrier density to substantially recover from said depletion, wherein the drive means is arranged to produce excitation pulses of sufficiently high pulse energy that the excitation substantially completely depletes the established internal carrier density, and wherein the drive means is such as to control the durations of said excitation pulses to be substantially less than the time the amplifier takes to recover from said depletion, thereby in use to generate said optical output pulses in the period during which the established internal density is being depleted. A source as claimed in claim 4, wherein the series of excitation pulses define bit periods, and the drive means includes means connected to receive input data signals to convert the input data signals to return-to-zero format in which data pulses representing binary "1" values occupy only a small proportion of said bit periods. A source as claimed in claim 4 or claim 5, wherein the amplifier comprises a travelling wave amplifier. A source as claimed in claim 6, wherein the gain ripple of the amplifier is at most 3dB. A source as claimed in any one of claims 4 to 7, wherein the amplifier comprises a laser having first and second facets, first and second single layer refractory oxide anti-reflection coatings being disposed on the first and second facets, respectively. A source as claimed in any one of claims 4 to 8, wherein the drive means comprises means to generate the series of excitation pulses with a pulse repetition interval T r of about 10ns and pulse durations of about 40 ps, and the amplifier has a peak saturation power of about 1mw and produces peak pulse power of the order of at least 250mW. A source as claimed in any one of claims 4 to 9, wherein the drive means comprises means to generate the series of excitation pulses with pulse durations of at most one tenth of said pulse intervals. A source as claimed in any one of claims 4 to 10, wherein the output pulse duration is at most one half of the carrier lifetime. A source as claimed in any one of claim 4 to 11, wherein the output pulse duration is at most one third of the carrier lifetime. A source as claimed in any one of claims 4 to 12, wherein the amplifier is arranged to receive input data in return-to-zero format. A source as claimed in any one of claims 4 to 13, wherein the amplified output pulses comprise a return-to-zero format data stream. A source as claimed in claim 14, wherein the return-to-zero data format comprises signal pulses of a duration less than half of the bit period. A source as claimed in claim 15, wherein the pulses are at the beginning portion of a period.

Inventors: MARSHALL IAN WILLIAM, O'MAHONY MICHAEL JOHN, MARSHALL, IAN WILLIAM, O'MAHONY, MICHAEL JOHN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: H01S3
IPC codes: H01S5, H01S3, G02F1