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Description  |
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TECHNICAL FIELD
This invention relates to mehods which detect electrical or optical changes
in semiconductor lasers and external cavity lasers.
BACKGROUND OF THE INVENTION
There are numerous applications in which it is desirable to detect small
changes, either electrical or optical, in the operating parameters of
semiconductor lasers. Measurements of these parameters, as well as perhaps
other parameters, are typically carried out with the laser operating at
both fixed temperature and output power using direct detection of the
laser output. Fixed power is typically maintained with automatic power
control (APC).
Fixed temperature and power operation is also used with the light source in
an optical time domain reflectometer which determines the distance from
the light source to, for example, a reflecting surface in an optical fiber
by measuring the elapsed time between pulse emission and return. In this
application, changes in the operating parameters are not measured.
An important application of operating parameter measurements is the
determination of semiconductor laser reliability or lifetime. While
lifetime measurements are important for virtually all laser applications,
they are especially important for optical communications applications,
such as submarine cables, where laser replacement is both difficult and
expensive.
Lifetime measurements typically rely on a technique, called accelerated
aging, which operates a semiconductor laser for a prolonged period of time
at a temperature elevated with respect to the normal operating
temperature. Changes in the electrical input power of the laser are
detected as constant optical output power is maintained. While perfectly
adequate for many purposes, this technique suffers from the drawback that
the degradation mechanism operative at the elevated temperature need not
be the degradation mechanism operative at the intended use temperature.
There is, therefore, an element of uncertainty in the lifetime results
obtained.
The measurement of semiconductor laser parameters is made still more
difficult by the discovery that nonlinear optical cavity effects produce
instabilities in the laser APC system. Thus, the accuracy of the parameter
measurements is limited by the control system's stability. The control
system has equilibrium points which are the solutions to the closed loop
control system. An equilibrium point is considered stable if the system
ultimately approaches that point. The concept of stability thus means, in
practical terms, the behavior of the control system in the neighborhood of
an equilibrium point. However, if the entire system has a single nonlinear
element, there may be regions in which there is no equilibrium point. It
should also be noted that some lasers may fluctuate between multiple
equilibrium points and reduce the signal to noise ratio. For example,
there may be discontinuities in the light output versus current
characteristic curve. This condition is made worse when, as inevitably
happens, a small perturbation disturbs the system equilibrium and causes a
temporal discontinuity in the parameter measurement. Thus, the precision
of the measurements is also limited by one's ability to distinguish
between instabilities in the device under test and the control system.
Uncertainties in the parameter measurements and the projected lifetimes
inevitably result.
SUMMARY OF THE INVENTION
It has been found that small electrical and optical changes in
semiconductor lasers may be detected with a modulation technique which
periodically varies the wavelength of the output radiation by varying an
operating parameter, such as the temperature, of the laser while
maintaining constant optical output from a first laser face, thereby
producing a change in, for example, the output wavelength and threshold
current of the laser. The constant optical power is maintained by, for
example, automatic power control. The output from the first or second
laser face may encounter an optical cavity in the optical circuit. A
periodic temperature modulation introduces amplitude and phase modulation
into the semiconductor laser output resulting from such an optical cavity.
A low pass filter rejects the phase modulation and the remaining amplitude
modulation signal is used to recover the relevant signal, that is, the
signal indicating changes in either or both the electrical or optical
characteristics of the laser. The method may be used for operating
temperature determinations of laser lifetimes and for precise
determinations of external optical cavity dimensions, that is, distances
from a laser output face to reflecting surfaces.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram illustrating apparatus suitable for the practice
of the method of this invention; and
FIG. 2 plots laser output from the optical cavity vertically in arbitrary
units versus the temperature of the laser horizontally in arbitrary units.
DETAILED DESCRIPTION
FIG. 1 is a schematic representation in the form of a block diagram of
apparatus suitable for the practice of this invention. The apparatus
comprises light source 1, temperature control unit 3, power control unit 5
and detection system 7 which is optically coupled to the light source by
means of, for example, optical fiber 9. As is evident, there is a feedback
loop between the power control unit and the light source. The power
control unit has means for monitoring the light output from a first face
of the laser and adjusting the current delivered to the laser to maintain
the light output from the first face at a constant level. Automatic power
control (APC) will maintain the optical output power constant. APC is well
known to those skilled in the art and need not be described in more
detail.
The light source is typically a semiconductor laser although other types of
lasers may be used if desired. The laser need not be a single longitudinal
mode laser, that is, a single frequency laser, as the highest power mode
will make the dominant contribution to the detected amplitude if a
multilongitudinal mode laser is used. More precisely, the method of this
invention will work with a multiple longitudinal mode output laser as the
method relies on the motion of the centroid of the lines rather than the
motion of individual lines.
The temperature control unit is also well known to those skilled in the art
and may comprise, for example, a thermoelectric unit which periodically
varies the temperature of the laser. Further description is not required.
The selection of a suitable period, as well as amplitude, will be
discussed later.
The temperature control unit is used to produce changes in the amplitude of
the light from the cavity by producing changes in the wavelength of the
light from the laser. It is possible to vary other parameters, such as
current, which also produce variations in the wavelength of the light. The
latter parameter produces smaller variations in the wavelength than does
the former parameter. Of course, in this embodiment, the temperature
control unit becomes a current control unit.
The detection system includes a photodetector and electronics to do the
necessary processing of the electrical signal generated by photons being
absorbed in the photodetector. The signal processing will be described
later. Details of the system will be readily known to those skilled in the
art.
Any well known optical fiber or other type of waveguide may be used to
optically connect either face of the source to a detection system. In all
embodiments, the source, the detection system and the APC system are
coupled through optical circuits that may include an optical cavity which
may be formed in, for example, free space. Free space is a desirable
cavity medium because the cavity is easily formed in the free space
between the laser and the optical fiber. Additionally, the distance from
the light source to any reflecting surface within the laser package may
form a cavity. The method of this invention can be used to accurately
measure the distance from the light source to such a reflecting surface
assuming the proper conditions are satisfied.
FIG. 2 shows the light output from the cavity plotted vertically in
arbitrary units versus the laser temperature horizontally, also in
arbitrary units.
The range of temperature modulation is determined by the change of
wavelength with temperature and the magnitude of the temperature change
that is easily obtained. Of course, the temperature should change by an
amount sufficient to change the wavelength by an amount sufficient to
produce a significant change in the intensity of the radiation from the
cavity. The rate of temperature change is limited by practical
considerations such as the need to obtain data in a reasonable amount of
time and the desire to avoid extraneous effects that might arise if
portions of the laser package were at different temperatures.
The power control unit (APC) is an automatic system which uses a
photodetector to measure the light output from the first, i.e., back, face
of the laser to start the feedback loop. An error signal is derived at the
input of the power amplifier, which drives the laser, by comparing the
laser output power, as measured by the photodetector, to a reference
signal. The control system then adjusts the laser bias current, generated
by the amplifier, to minimize the error signal. The desired operating
point is established when the minimum error signal is obtained.
The controllability of the nonlinear system is improved by the introduction
of the small carrier modulation. This is accomplished according to the
present invention by periodically varying the laser operating wavelength
by varying, for example, the laser temperature around a nominal
temperature. This modulation technique has the advantage of not only
varying the laser wavelength but also modulating both the threshold
current and, in some embodiments, the fiber-laser alignment or the
physical geometry of other optical cavities. A variation of typically only
several degrees C. is sufficient to enable such parameters as the
threshold current, optical wavelength, and laser-fiber alignment, if
present, to be usefully studied. The wavelength of the emitted radiation
depends upon other parameters, such as current, which could be
periodically varied with the temperature remaining constant.
The signal which must be ultimately recovered is the variation in the
operating parameter of the laser package being studied. For long term
lifetime studies, the detection system must be sensitive to the long term,
gradual degradation of these operating parameters. The desired signal may
be recovered by using a low pass filter at the modulation frequency to
eliminate harmonic components produced by phase modulation. This method
essentially recovers the baseband portion of the modulated output signal.
The carrier signal is then removed by linear superposition using the
temperature signal as a local oscillator. This effectively provides notch
filtering cation around the carrier frequency. Of course, the phase of the
local oscillator and modulated signal must remain synchronized for the
superposition to be effective. Because the filter has a phase delay at the
carrier frequency, the local oscillator signal is processed by an
indentical filter to maintain phase synchronization.
The signal is desirably processed with digital signal processing
techniques. A recursive filter may be constructed which will replicate,
e.g., Butterworth, Chebyshev, Elliptic or Bessel characteristics. Details
will be readily apparent to those skilled in the art and need not be
discussed in further detail. The desired amplitude response can be readily
achieved.
The continuous time signals are converted to a digital time series by
sampling. The ideal sampling function consists of an infinite series of
equally spaced impulses or Dirac delta functions. The output of the
sampler is thus a time series of equi-distant pulses with amplitudes equal
to the value of the function being sampled at that time. Distortion is
avoided if the original signal is band limited in the sampling interval is
chosen to be at least twice the band limited frequency. This is the
Nyquist criteria and specifies the minimum sampling rate.
In addition to lifetime studies, the method of this invention is also
useful in measuring small distances as will be explained by the following
example.
The resonance for a cavity of length d coupled to an optical source of
wavelength .lambda. is given by
d=.lambda.(.lambda.+.DELTA..lambda.)/2.DELTA..lambda.. (1)
If it is now assumed that .DELTA..lambda. is much less than the wavelength
.lambda., equation (1) simplifies to
d=.lambda..sup.2 /2.DELTA..lambda.. (2)
Now if it is assumed that .DELTA..lambda. may be approximated by
.DELTA..lambda.=(.delta..lambda./.delta.T).tau. where
.delta..lambda./.delta.T is the temperature-wavelength coefficient of the
laser source, .tau. is the degrees C. per cycle between amplitude maxima.
The cavity length may then be represented by
d=(.lambda..sup.2 /2.tau.).delta.T/.delta..lambda.. (3)
For a laser emitting at a wavelength of 1.3 microns, .delta..lambda./.tau.T
is typically approximately 0.75 angstroms per degree C. and the wavelength
is 13,000 angstroms. Thus, in centimeters, d=1.127/.tau.. If .tau. is 0.1
degrees C., d will be approximately 11 centimeters while if .tau. is
approximately 10 degrees C., d will be approximately 0.1 centimeter. Thus,
measurement of the space between amplitude maxima permits determination of
these distances or changes in distance with accuracy. A similar analysis
will be readily done by those skilled in the art for other embodiments
such as using a periodic current variation to vary the wavelength of the
emitted light.
Thus, choice of other lasers with different characteristics or use of other
modulation schemes will permit the method to measure either larger or
smaller distances accurately.
Another application of temperature modulation examines the Fabry-Perot
reflections of individual laser packages to determine their suitability
for high speed, near 0.5 Gbit/sec or even greater, optical transmission
systems. Optical feedback to the laser may produce intensity variations
that result in a power penalty at the receiver. The Fabry-Perot
reflections degrade system performance, and those packages with large
amplitude variations, induced by reflections, may be identified. Thus, a
static measurement is useful in characterizing dynamic performance
characteristics.
Small mechanical displacements may be measured by forming a Fabry-Perot
etalon or interferometer cavity at, for example, the end of a fiber.
Scanning wavelength by temperature modulation produces intensity
variations. However, if the cavity dimensions change due to pressure,
vibrations, etc., the amplitude maxima shift to new positions when the
bias current is plotted versus temperature. In this case, APC is used, but
the change in bias current with temperature is measured.
The following considerations assist in understanding this application. The
laser is operated with temperature modulations using APC. Amplitude
variations in control current are measured. It can be shown that these
amplitude variations are given reasonably accurately by Airy's formula:
##EQU1##
where i and i.sub.max are the detected and maximum photocurrent,
respectively; F=4R/(1-R).sup.2 where R is the reflectance; and
.delta.=4.pi.nd cos .theta./.lambda. where n is the refractive index, d is
the spacing, .theta. is the angle of incidence and .lambda. is the
wavelength. For a hypothetical, but realistic situation in which R is
approximately 1 percent, i/i.sub.max will be approximately 4 percent, an
easily detected change.
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