Colliding pulse mode-locked fiber ring laser using a semiconductor saturable absorber

A CPM mode-locked fiber ring laser including an ion-implanted semiconductor saturable absorber providing sub-picosecond pulses at rapid repetition rates. The laser provides compact, turn-key capability and is polarization insensitive.

FIELD OF INVENTION 
The invention relates to lasers and more particularly to a laser assembly 
providing a colliding pulse mode-locked (CPM) fiber ring laser using a 
semiconductor saturable absorber. 
BACKGROUND OF INVENTION 
Optical fiber systems need reliable sources of short light pulses at high 
repetition rates to carry information through optical fibers. The 
information carried ranges from telephone conversations, to television 
signals, to digital computer data. Sources of short light pulses are also 
needed for lightwave instruments and generally in fields of scientific 
research. Currently available picosecond and sub-picosecond duration 
lasers require highly skilled personnel to operate the lasers because of 
the great number and complexity of optical adjustments and alignments. 
There is a need for an easily operable laser capable of producing 
sub-picosecond light pulses. 
Short optical pulses are essential for testing the time response of fiber 
optic communication systems. The time response is related to the maximum 
rate of transmitting information. As the speed of telecommunications 
systems increases, the need for sub-picosecond testing impulse source 
becomes more acute. 
Mode-locking is an important technique for generating ultrashort pulses. 
Mode-locking causes the oscillation energy of the laser to be condensed 
into a packet in time domain that travels back and forth inside a laser 
cavity--the consequences of fixing the phases of the longitudinal modes of 
a laser. It is well known that mode-locked lasers can produce short 
optical pulses on the order of sub-picosecond pulses or shorter. 
Mode-locking occurs spontaneously in some lasers if the optical path 
contains a saturable absorber (an absorber whose absorption decreases with 
increasing optical intensity). This is the method used to induce 
mode-locking in continuous dye lasers, due to the fact that a dye will 
absorb less power from a mode-locked train of pulses than from a random 
phase oscillation of many modes since the first form of oscillation leads 
to the highest possible peak intensities (for a given average power of the 
laser) and is attenuated less severely as a consequence. For a general 
discussion see Yariv, A., Quantum Electronics, 2nd Ed. New York: Wiley 
(1975); Optical Electronics, 3rd Ed. New York: Holt, Rinehart & Winston 
(1985). 
Colliding pulse mode-locked (CPM) dye lasers have been reported to have 
produced light pulses of 1 picosecond or shorter. (See Fork et al., 
"Generation of optical pulses shorter than 0.1 psec by colliding pulse 
mode locking" Appl. Phys. Lett. vol. 38, No. 9, pp 671-2 (1961)). Ring 
lasers have the unique capability of oscillating simultaneously or 
independently in either of two distinct counter-propagating directions. 
Colliding pulse mode-locking (CPM) uses two synchronized 
counterpropagating pulses interacting in a thin saturable absorber to 
produce a short pulse. 
Since becoming commercially available, erbium-doped fiber has become the 
preferred gain medium for generating short optical pulses in actively and 
passively mode-locked lasers. Mode-locking of erbium fiber lasers has been 
demonstrated using active modulators, saturable absorbers, polarization 
switching, and non-linear amplifying loop mirrors. 
Saturation (the condition in which the population difference has reached a 
steady state) is very important to the laser theory. Saturation of the 
inverted population difference and hence the gain in an amplifying laser 
medium is what determines a laser's power output. When a laser oscillator 
begins to oscillate, the oscillation amplitude grows at first until the 
intensity inside the cavity is sufficient to saturate and therefore reduce 
the laser gain. Steady-state oscillation then occurs when the saturated 
laser gain becomes just equal to the total cavity losses, so that the net 
round trip gain is exactly unity. Gain saturation is thus the primary 
mechanism that determines the power level at which a laser will oscillate. 
Siegman, Lasers, University Science Books, p 207-8 (1986). 
It is well known that semiconductors have two categories of properties: 
electronic and optical. (See Understanding Lasers, ch 8, IEEE, 1992.) For 
a fiber laser using a semiconductor as a saturable absorber, optical 
properties are important. The key parameters are bandgap and carrier 
lifetime. Bandgap is the energy spacing between the conduction band and 
the valence band and represents the energy (frequency) of light which just 
begins to produce electrons and holes in the semiconductor. Near the edge 
of the bandgap the number of available states for electron/hole production 
is limited. The word "gap" is used to describe the lack of energy levels 
for electrons and holes in between the conduction band and the valence 
band if the light frequency is too low. 
In semiconductor material, electrons are in the valence bands under normal 
circumstances. When light with energy level equal to or greater than the 
semiconductor's band gap energy encounters the semiconductor, the light 
energy is absorbed, causing some electrons to transit to the conduction 
band and create electron-hole pairs. The electron-hole pairs eventually 
decay; the average lifetime is termed the "carrier lifetime." 
There are two kinds of decay processes: radiative and non-radiative. In the 
radiative decay process, an electron drops back from the conduction band 
to valence band with a photon emitted at a frequency proportional to the 
band gap energy. The radiative decay process has a time constant of 
several nanosecond (10.sup.-9 S) which is intrinsic to the particular 
semiconductor material. 
Non-radiative decay is a process by which an electron and a hole combine 
without generating light. Common mechanisms for non-radiative 
recombination are defects and impurities in a semiconductor. The time 
constant of non-radiative process depends on the density of the impurities 
and defects and can be shorter than 1 picosecond. Semiconductor saturable 
absorbers use the mechanism of absorption saturation. As mentioned 
previously, when a semiconductor absorbs light, the electrons transit from 
valence bands to conduction bands thus creating electron-hole pairs. When 
incident light intensity is high, there are so many transitions occurring 
that the valence band is almost empty and the conduction band is full, a 
semiconductor's ability to absorb light becomes weakened. This phenomena 
is known as the saturable absorber effect. The properties of a saturable 
absorber depend on its carrier lifetime. The shorter the carrier lifetime, 
the faster the decay of the electron-hole pairs, and the faster the 
recovery of the saturable absorber. The saturable absorber effect can be 
used to make a mode-locked laser. Due to loss reduction when light 
intensity is high, a laser with a saturable absorber inside tends to 
mode-lock to produce short pulses with high instantaneous intensity rather 
than continuous wave (CW) lasing of comparatively low intensity. 
Recently, fiber lasers with semiconductor saturable absorbers have been 
reported. See Reddy et al., "A Turnkey 1.5 micrometer Picosecond Er/Yb 
Fiber laser", Optical Fiber Communication Conference, PD17, pp 71-4 
(1993). Reddy et al. report a fiber laser employing erbium-doped fiber 
pumped by a Nd:YLF (1.053 um) microlaser pumped by a diode laser. 
Mode-locking was achieved using an InGAsP saturable absorber deposited on 
a InP substrate. 
The Reddy et al. laser was claimed to provide self-starting mode-locking 
and to operate insensitive to polarization drift in the fiber. Therefore, 
the laser might be made to be a "turnkey" operation: that is to say, a 
laser which will operate at the turn of a key. However, several serious 
shortcomings still exist, leaving unmet commercial needs with respect to 
short pulse lasers. 
The necessary semiconductor saturable absorber is not commercially 
available. In order to configure a mode-locking fiber laser, the 
semiconductor saturable absorber must have both the correct band gap and 
carrier lifetime. Although semiconductor saturable absorbers can be grown 
using MBE (molecular beam) or MOCVD (metal organic chemical vapor 
deposition), the carrier lifetime of the semiconductor material grown by 
these machines is usually too long to enable Er-doped fiber laser 
mode-locking. 
Short carrier lifetime has been achieved using low temperature MBE or MOCVD 
technique, however the techniques are difficult to reproduce and to 
control. It is fair to say that the making of semiconductor saturable 
absorbers for mode-locked fiber lasers or other solid state lasers has 
been perceived as a "black art" of sorts! Thus, the need remains for a 
commercially available semiconductor saturable absorber to enable the 
commercial availability of turnkey mode-locking lasers. 
Further needed is a commercially exploitable means of ion implantation. Ion 
implantation on semiconductor saturable absorbers is used to control the 
carrier lifetime of the electron-hole pairs. Without modifications, 
semiconductor materials such as InGaAsP/InP or GaAlAs/GaAs have natural 
carrier lifetimes of 1 to 8 nanosecond (a nanosecond equals 10-9 second). 
A carrier lifetime of this order is too long to produce the short optical 
pulse in an Er-doped fiber laser. One way to shorten the carrier lifetime 
in the semiconductor material is to use ion implantation. Ion implantation 
produces variations or "defects" in the semiconductor material. The 
defects serve as recombination centers for electron-hole pairs. By 
controlling the dosage and energy of ion implantation, the carrier 
lifetime can be reduced to the value required. The typical required value 
would be 1 to 100 picoseconds (a picosecond equals 10.sup.-12 second). Ion 
implantation is a better technique for producing defects in semiconductor 
saturable absorbers than the techniques of using low temperature MBE or 
MOCVD. Ion implantation allows the defect density and distribution to be 
well-controlled through varying the implantation dosage and energy. 
Moreover, low temperature growth of semiconductor saturable absorbers is 
an extremely difficult process. Most MBE or MOCVD machines are normally 
used to produce low defect material. Thus, low temperature operation for 
the creation of high defect material is "abnormal operation" of MBE or 
MOCVD machinery and, as such, requires extensive calibration. 
Lamprecht et al. studied time-resolved,luminescence in proton bombarded InP 
(indium phosphate) in order to measure the lifetime of photoexcited 
carriers as a function of the damage dose. (See Lamprecht et al., 
"Ultrashort carrier lifetimes in H+Bombarded InP", Appl. Phys. Lett. 59 
(8), pp 926-928, Aug. 19, 1991). Results were interpreted as meaning that 
the decrease in lifetime was a direct consequence of the defects of 
trapping and recombination centers produced by the bombardment. 
Photoexcited carriers in the proton bombarded InP demonstrated ultrashort 
lifetimes down to 95 femtoseconds. 
Van der Ziel et al. reported using ion implantation to make a mode-locked 
semiconductor laser wherein the region of saturable absorption was 
introduced by proton bombardment. See "Generation of subpicosecond pulses 
from an actively mode-locked GaAs laser in an external ring cavity," Appl. 
Phys. Lett. 39(11), pp 867-869, Dec. 1, 1981. The treatment of inducing 
saturable absorption through proton bombardment converts initially stable 
emission from untreated samples (prior to bombardment) to a pulsating 
output from treated samples. Van der Ziel's external cavity consisted of a 
collimating lens and a reflecting mirror. Proton bombardment of the mirror 
facet introduced saturable loss, yielding sub-picosecond pulses in 
mode-locked semiconductor lasers. 
Although a mode-locked semiconductor laser using ion implantation technique 
was demonstrated by van der Ziel et al., semiconductor lasers have a short 
gain recovery time (on the order of a nanosecond, which is 10-9 seconds). 
Mode-locking involves gain saturation by the semiconductor control medium 
and loss saturation by the saturable absorber. 
There still remains a need for mode-locking lasers with much slower gain 
recovery times such as an Er-doped fiber amplifier which has gain recovery 
time of 10 millisecond (a millisecond being 10.sup.-3 seconds). Moreover, 
there still remains an unmet need for an easy to operate (i.e. self 
starting and polarization insensitive), compact laser capable of producing 
short optical pulses at high repetition rates. 
SUMMARY 
The present invention provides a colliding pulse mode-locked (CPM) fiber 
ring laser which is capable of providing short optical pulses (on the 
order of 1-2 picoseconds or less) at high repetition rates. The invention 
provides compact, easy to operate CPM fiber ring laser capable of short 
optical pulses with repetition rates of several tens of megahertz. The CPM 
fiber laser can have a repetition rate which is four times faster than a 
linear laser containing the same fiber length and twice the rate os a 
unidirectional ring laser of the same length. This higher repetition rate 
is achieved by summing timing the outputs from both sides of the output 
coupler. Moreover, the CPM reduces chirping of the pulses so that the 
output pulsewidth can be shorter. The invention taught herein provides a 
polarization insensitive CPM fiber laser which can be as compact as a deck 
of playing cards--a tremendous reduction in size over CPM dye lasers which 
occupy space equivalent to a banquet table or greater. CPM dye lasers are 
open beam and require mirrors to bend the light beam into a ring. The 
fiber is easily wrapped into a ring and inherent to the fiber is a high 
degree of confinement of light. Further provided is a superior technique 
of ion implantation in semiconductor saturable absorbers. 
Briefly and in general terms, a CPM fiber ring laser according to the 
invention includes a ring consisting of single mode fiber and a portion of 
erbium-doped fiber, a semiconductor saturable absorber, an output coupler, 
a wavelength division multiplier, and a plurality of lenses. 
A more complete understanding and appreciation of the aims and objectives 
of the present invention may be achieved by referencing the following 
description of the preferred embodiment and the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings for the purposes of illustration, the invention is 
embodied in a ring which includes a semiconductor saturable absorber (FIG. 
1, 30). There has been a need for a laser which generates short optical 
pulses with high repetition rate but which is also compact and easy to 
use. 
In accordance with the invention, a CPM fiber ring laser is assembled so as 
to include a semiconductor saturable absorber (FIG. 1, 30). The resulting 
laser does not require an isolator or mirror (which lowers manufacturing 
costs). The laser is compact due to the ring configuration; it will 
produce sub-picosecond optical pulses with repetition rates of several 
tens of megahertz. 
As shown in FIG. 1, a CPM fiber ring laser generally 12, according to the 
invention, includes a fiber loop 14, consisting of both single mode fiber 
16 and erbium-doped amplifier fiber 18. The fiber loop 14 has an output 
coupler 20, a wavelength division multiplexer coupler 22, a filter 24, a 
first and second lens 26, 28 and a semiconductor saturable absorber 30. 
Although the filter 24 helps to stabilize the laser, but is not essential 
to the functioning as the laser will operate without a filter. Likewise, 
lenses 26, 28 are not essential. It is apparent that FIG. 1 is not to 
scale. 
The CPM fiber laser works as follows. As with most lasers, the optical 
pulses in a CPM fiber laser are started from spontaneous emission (or 
noise) from the fiber amplifier. The pulses are amplified by the fiber 
amplifier; the saturable absorber shapes the pulses. Since the loss is 
minimal when the two counterpropagating pulses collide on the 
semiconductor saturable absorber, eventually the two counterpropagating 
pulses become the main lasing mode of the CPM fiber laser. 
The colliding pulse mode-locking configuration is especially useful for 
fiber lasers. Wrapping fiber into a ring configuration is easily done and 
is often preferable to a linear cavity due to the fiber's ability to 
confine light to a high degree. The colliding pulse effect enhances the 
pulse shaping mechanism of the saturable absorber. 
The above described components, with the exception of the saturable 
absorber, are commercially available. The erbium-doped fiber amplifier can 
be purchased from AT&T; approximately 8 meter with Er dopant concentration 
of 300 ppm fiber is used in the invention described herein. The band pass 
filter has a center wavelength of 1553 nm and a bandwidth of 5.0 nm is 
available from Omega Optical Inc. The wavelength division multiplexer (980 
nm/1550 nm) is available from Amphenol Corp., as are output couplers 
(ratio 17%). The GRIN lenses (0.22 pitch) are available through NSG 
American, Inc. 
The semiconductor saturable absorber can be grown using an MOCVD machine 
(available from Swan, Inc.) and using substrate available from Sumitomo, 
Inc. On a substrate of InP, for example, a 2.0 (+/-0.2) micrometer 
epitaxial growth of InGaAsP is desirable, with a bandgap of 1.567 
micrometers (+/-.030 micrometers). Depending on factors such as Er-doped 
fiber length, Er-doping concentration, and loss in the cavity, the 
epitaxial growth layer may be as thick as 6 micrometers. 
An antireflection coating is applied to both side of the saturable absorber 
using Si.sub.x N.sub.x, at a wavelength of 1.55 micrometers. The 
antireflection coating service may be obtained commercially through Denton 
Vacuum, Inc. 
The CPM configuration using a semiconductor saturable absorber provides an 
advantage in that it is not sensitive to polarization disturbance to the 
fiber as discovered through experimentation. It has been found that the 
invention, a CPM fiber with a saturable absorber, can tolerate large 
amounts of polarization disturbance. Through a comparison of colliding 
pulse configuration with that of unidirectional ring configuration, it was 
found experimentally that the CPM configuration can withstand much larger 
polarization disturbances. 
FIG. 2 shows a cross section of a saturable absorber 30, including the 
active layer 34 and the substrate layer 32. The active layer 34 is 2.0 
micrometers thick and the substrate layer 32 is 0.3 millimeters thick. The 
antireflection coating on each side 36 of the saturable absorber 30 
reduces reflection to less than 1% per surface. 
The absorptive layer is comprised of a combination of materials selected 
form group III and IV of the Periodic Table of the Elements. Preferred 
combinations include InGaAsP, InPAs and InGaAs. The substrate provides 
support to the absorptive layer and preferred combinations include InP and 
GaAs. 
Higher implantation dosages on a saturable absorber tends to produce a 
laser generating a poor quality pulse, which has pedestal. Lower 
implantation dosages on the saturable absorber tends to produce wider 
pulses. The preferred protocols are those which produce a saturable 
absorber capable of short, pedestal free pulses. 
FIG. 3 depicts the optimum focusing scheme of the invention with respect to 
the relationship of the fiber ends 15 and the saturable absorber 30. The 
fiber ends 15 are polished at an angle of approximately 10.degree.. 
Between the fiber ends 15 are placed, in the optical path, the filter 24, 
the first GRIN lens 26 (0.22 pitch, .phi.3.0 mm), the saturable absorber 
30, the second GRIN lens 28 (identical to first GRIN lens 26). The optimal 
spacing between fiber end 15 and first GRIN lens 26 is represented in FIG. 
3 by 42 and in the preferred embodiment, 42 has the value of 7 mm. The 
spacing between first GRIN lens 26 and the saturable absorber 30 is 
represented in FIG. 3 by 44 and in the preferred embodiment 44 has the 
value of 2 mm. Likewise the distance between the saturable absorber 30 and 
the second GRIN lens 28 is represented by 46 and in the preferred 
embodiment 46 has the value of 2 mm. The lens configuration produces a 3:1 
image reduction on the saturable absorber. 
It is possible to insert the saturable absorber directly in the optical 
path without using a lens. This can be accomplished by lapping down the 
substrate layer of semiconductor saturable absorber to less than 100 
micrometers and then placing the two fiber ends in direct contact with the 
saturable absorber. Since the saturable absorber is so thin, efficient 
coupling between the two fiber ends can be achieved without using lenses. 
The colliding pulse configuration reduced the sensitivity of the 
mode-locked fiber laser to the polarization variation. Two configurations, 
shown in FIG. 4A and FIG. 4B, were compared. With the exception of the 
isolator 40 in FIG. 4B, the configurations depicted in FIG. 4A and 4B are 
identically constructed, consisting of a fiber loop 14 (including both 
single mode fiber 16 and erbium-doped fiber 18) and associated with the 
fiber in the optical path an output coupler 20, a wavelength division 
multiplexer coupler 22, a polarization controller 38, a filter 24, a pair 
of GRIN lenses 26, 28 and a semiconductor saturable absorber 30. The 
configuration depicted in FIG. 4A can tolerate large polarization 
variation. The polarization controller 38 consists of three fiber loops. 
Each of the fiber loops simulates a quarter wave plate. The adjustment of 
polarization is accomplished by changing the orientation of the fiber loop 
in a 0 to 180 degree range. When FIG. 4A laser is mode-locked, stable 
mode-locked output can be maintained when the polarization controller is 
in the 0 to 120 degree range. 
The FIG. 4B laser configuration can tolerate much less polarization 
variation. When the FIG. 4B laser is mode-locked, stable mode-locked 
output can be maintained only when polarization controller is varied 
within 5 degrees (as compared with 120 degrees in laser configuration 4A). 
Thus, the CPM (colliding pulse mode-locked) configuration tolerates much 
more polarization variation than a unidirectional ring configuration. 
From the foregoing it will be appreciated that the present invention 
provides a method and apparatus for generating short optical pulses on the 
order of 1-2 picoseconds or less, with repetition rates in the tens of 
megahertz, which is self-starting and polarization insensitive. 
A laser system that embodies the principles of the invention is less 
sensitive to polarization variation, is capable of higher repetition 
rates, requires fewer components and, therefore, lowers production costs. 
For these and other advantages provided by the invention herein, it is 
superior to lasers currently available in its ease of use. 
It will be apparent that different working embodiments in a wide range can 
be formed without deviating from the spirit and scope of the present 
invention. Therefore, the present invention is not restricted by the 
specific embodiments described and illustrated herein except as being 
limited in the appended claims.