Frequency modulated laser diode

A method of modulating the output of a solid state diode laser by directly straining the laser chip. The chip is mechanically coupled to a transducer such as a piezoelectric crystal in such a manner that a dimensional variation in the transducer is translated into a corresponding variation in the chip. It is contemplated that the modulated output will have application in communication systems and in particular a hydrophone system.

The invention described herein may be manufactured, used or licensed by or 
for the government of the United States of America for governmental 
purposes without payment to us of any royalties therefor. 
The invention relates to a method whereby the output of a solid state diode 
laser may be modulated by directly straining the laser chip. The invention 
contemplates the use of a solid state diode laser source whose output is 
modulated by the aforementioned method in a communication system in which 
the modulated laser output is transmitted to a remote detector through the 
atmosphere or via an optical fiber. More particularly, the method is 
adaptable for use in a hydrophone system. 
The small size, relatively low power requirements, environmental ruggedness 
and compatability with known detectors, renders the solid state diode 
laser particularly well suited for use as a radiation source in line of 
sight communication systems and systems involving optical fibers as the 
transmission medium. Information concerning structures and operational 
characteristics of solid state diode lasers may be found in Procedings of 
the IEEE Vol. 64, pp. 1512-1240, October 76; (M. B. Panish). 
A fiber-optic hydrophone using a Fabry-Perot sensing cavity is described in 
a co-pending patent application to P. G. Cielo and G. W. McMahon, "Stable 
fiber-optic hydrophone", Canadian application Ser. No. 333,603. In that 
system a light beam is introduced into an optical fiber containing a local 
cavity, whose length can be varied by a piezoelectric transducer, and 
sensed by a remote cavity of equal length. The local cavity length is made 
to follow the length of the sensing cavity by a feedback circuit, thus 
retrieving a signal which is a function of the pressure to which the 
remote cavity is subjected. 
A diode laser is a particularly appropriate choice as the optical source, 
because of the aforementioned features, namely small size, suitability for 
fiber coupling, convenient emission wavelength and low cost. However, in 
the system of application Ser. No. 333,603 the presence of two passive 
cavities along the fiber introduces a certain amount of attenuation of the 
optical beam, particularly if high reflectivity reflectors are used for 
the cavities. Moreover, the demodulation curve for a double-cavity system 
is less sharp and provides a lower index of modulation than a 
single-cavity demodulation curve, which would be obtained if a single-mode 
laser and a single Fabry-Perot cavity were used. Single mode lasers are 
presently available but they are expensive and easily detuned. 
The system proposed herein introduces a new technique for frequency 
modulating the output of a diode laser, so that in a hydrophone 
application only a single-cavity demodulator is required. The local cavity 
is replaced by the laser cavity itself, which is an active cavity and thus 
introduces no attenuation in the transmitted optical beam. Both a 
single-mode or multi-mode diode-laser can be used, the demodulation curve 
corresponding in both cases to the sharp, single-cavity demodulation 
curve. 
Thus, according to one aspect of the present invention there is provided a 
system for modulating the output of a diode laser of the type including a 
Fabry-Perot laser cavity wherein the diode-laser is mechanically coupled 
to a dimensionally variable transducer such that a dimensional variation 
in the transducer responsive to an external signal results in a 
dimensional variation in the laser cavity. 
According to a second aspect there is provided a method of modulating the 
output of a solid state diode laser, the diode laser including a 
Fabry-Perot laser cavity comprising the steps of mechanically coupling the 
diode laser to a dimensionally variable transducer, and causing the 
transducer to vibrate by applying an external signal thereto, thereby 
causing the laser cavity to vibrate synchronously with the vibrating 
transducer. 
In one embodiment there is provided a communication system comprising a 
solid state diode laser and a remote sensor wherein the output of the 
diode laser is modulated synchronously with the vibration of a 
dimensionally variable transducer, the diode laser being mechanically 
coupled to the transducer and the transducer being made to vibrate by an 
external signal applied thereto. 
In accordance with a further embodiment there is provided a hydrophone 
system comprising a frequency modulated laser diode source, a length of 
optical fiber adapted to transmit the output of the modulated laser diode, 
an optical cavity sensing acoustic pressure fluctuations, a second optical 
fiber transmitting light from the sensor to the detector and a 
data-processing circuit that drives the laser modulating transducer and 
retrieves the required acoustical pressure signal.

FIG. 1 is an illustration of a typical solid-state diode laser chip. The 
diode laser consists of a multilayered parallelipiped of gallium arsenide 
and aluminum gallium arsenide grown as a single crystal. It is to be 
understood, however, that diode lasers of alternate configuration or 
materials may be used. A full metal electrode 10 contacts one of the 
larger faces of the chip and a strip electrode 11 contacts the opposite 
face. The two smaller faces of the chip, perpendicular to the strip 
electrode, are polished to form partially reflecting parallel mirrors, 
which make up the Fabry-Perot laser cavity. Laser light 12 emerges from 
one of the suitably doped layers of the chip when sufficient current is 
passed between the electrodes. 
The frequency band of the output from a diode laser depends on the band gap 
energy of the laser active region and is typically about 3.6 (10.sup.14) 
Hz for GsAs-AlGaAs lasers. Lasing can occur for frequencies near the 
center of this band whenever the length of the cavity (i.e. the mirror 
separation) is an integral number of half-wavelengths long. The allowed 
optical wavelengths are therefore given by 
EQU .lambda..sub.n =(2L.mu./n) 
where L is the mirror separation, .mu.is the reflective index of the 
optical wave guide (.perspectiveto.3.6) and n is an integer. Thus, the 
output spectrum of the laser is typically a band of several discrete 
frequencies with wavelengths corresponding to the above expression. When 
the length L of the laser chip is varied by longitudinally straining the 
chip, each of these frequencies will be varied in inverse proportion to 
the strain. 
FIG. 2 shows a diode laser chip which uses a slightly different contact 
design which may be advantageous for certain implementations of the 
present invention. Here the strip electrode 14 covers the entire upper 
surface of the chip but is allowed to contact the chip only along a narrow 
strip 15, being isolated from the remaining area by an insulating layer 
16, such as SiO.sub.2. 
Typical dimensions for a diode laser chip are 0.3.times.0.2.times.0.1 
millimeter. In operation, a direct current from a source of the order of 2 
volts is typically passed between the two electrodes. 
FIGS. 3 and 4 show one embodiment of the transducer system whereby the 
frequency of the laser output is modulated. The diode laser chip 20 is 
mechanically coupled via an electrically conducting material such as 
solder to one electrode 21 of piezoelectric crystal 22. In a further 
variation the chip may be coupled to the transducer by means of 
electrically conducting silver filled epoxy. A modulating voltage is 
applied across the piezoelectric crystal between one electrode 21 and a 
second electrode 23, causing the crystal to be strained in proportion to 
the applied voltage. There is a corresponding variation in the length of 
the laser cavity, because of the mechanical bond between the piezoelectric 
crystal 22 and the laser chip 20. This generates the required frequency 
modulation of the laser beam. 
Other components are also shown in FIGS. 3 and 4: A heat sink 25 of some 
good conductor such as copper is required to carry excess heat away from 
the laser chip. An insulating block 26 supports a terminal 27 which is 
connected via a wire 28 to the strip electrode 11 of the laser chip. 
For clarity, FIGS. 3 and 4 show a laser diode having the contact design of 
FIG. 1; however, because the laser active region is much closer to the 
strip electrode surface than to the opposite surface of the chip, a better 
modulation index will be achieved if the design of FIG. 2 is used and the 
surface 14 is bonded to the piezoelectric crystal. 
Differential thermal expansion between the transducer block and the chip 
can place high stresses on the bond joining the two components if their 
expansion coefficients are poorly matched. Gallium arsenide has a thermal 
expansion coefficient of about 5 (10.sup.-6) per .degree.C. (Cottam & 
Saunders, J. Phys. C: Solid State, Vol. 6, p 2105 ff, 1973). A 
piezoelectric transducer of lead zirconate titante (PZT-4) in the 
configuration of FIGS. 3 and 4 is fairly closely matched, having a 
coefficient of 3.8 (10.sup.-6) per .degree.C. 
FIG. 4A shows a configuration similar to that of FIGS. 3 and 4 but using a 
magnetostrictive transducer. The block 61 is made of laminated 
magnetostrictive material having its direction of maximum magnetostrictive 
strain in the same direction as the laser axis. A coil of wire 62, with 
its axis parallel to the laser axis, is wound around the block and the 
laser chip. An electrical current I is passed through the coil to provide 
the required modulating signal. The resulting magnetostrictive strain in 
the block causes a corresponding variation in the length of the laser 
cavity in the same manner as for the piezoelectric transducer. A heat sink 
is not shown and may not be required since most magnetostrictive materials 
are metallic and the block itself may provide an adequate heat sink. The 
magnetostrictive material is laminated to reduce eddy currents at high 
frequencies of modulation. This effect will ultimately limit the upper 
modulating frequency to lower values than for piezoelectric transducers. 
FIGS. 5 and 6 show a more complex implementation of the transducer system 
which provides better mechanical coupling of the piezoelectric strain to 
the laser chip and also improves the conduction of excess heat away from 
the chip. In this embodiment the laser chip 20 is sandwiched between two 
piezoelectric crystals 30 and 31 and bonded to the electrodes 32 and 33 of 
the two crystals. The heat sink 34 has a thin insulating layer 35 applied 
to one surface and is also bonded between the two electrodes 32 and 33. 
Hence the heat sink and insulating layer are of the same thickness as the 
laser chip with its electrodes. In this configuration a laser chip 
according to the form of FIG. 2 would be more desirable. The diode laser 
is driven by a direct current applied between electrodes 32 and 33. The 
modulating voltage is applied across the piezoelectric crystals via 
electrodes 32 and 33 and electrodes 37 and 38. The polarity of the 
piezoelectric crystals is chosen so that the piezoelectric stains are in 
the same direction. The small DC voltage between electrodes 32 and 33, 
required to drive the laser, is of no consequence to the modulating 
voltage. 
In operation of the invention according to this embodiment the laser light 
emerges from the device and is coupled to a "pigtail" of monomode optical 
fiber 39. The fiber core is aligned with the beam emerging from the laser 
and is held in a stable position by some suitable means such as a 
transparent thermosetting plastic 40. The plastic preferably has an index 
of refraction matched to that of the optical fiber, preventing secondary 
reflections from the end of the fiber. Because the index of refraction of 
aluminum gallium arsenide is very high (.perspectiveto.3.6), the laser 
cavity is not seriously affected by the thermosetting plastic. 
Yet another implementation of the invention is depicted in FIGS. 7 and 8 in 
which the lateral surfaces of the laser chip 20 are sandwiched between two 
piezoelectric crystals 41 and 42 and the electrode surfaces of the laser 
chip are bonded electrically and mechanically to copper terminal blocks 43 
and 44, which also form the heat sinks to carry excess heat away from the 
laser. The piezoelectric crystals have electrodes 45, 46, 47 and 48 on 
surfaces perpendicular to the laser beam direction, which, for many 
piezoelectric materials, will enhance the available piezoelectric strain 
along the laser beam direction. A thin insulating material 49 may be 
required between the heat sinks and the piezoelectric crystals so that the 
heat sinks do not shunt part of the electric field in the crystals. The 
diode laser configuration of FIG. 2 is again the most convenient for use 
in this implementation. Note that it is not necessary for the diode laser 
chip to be in close contact with the piezoelectric crystals since the 
terminal blocks will be strained and will transfer the strain to the laser 
chip. 
In order to simplify the diagrams, not all of the electrical connections 
are shown. These are understood to be accomplished using well-known 
techniques. The embodiment shown in FIGS. 5-8 illustrate coupling of the 
laser beam to an optical fiber for signal transmission of the modulated 
output to a remote sensor. It is to be understood that in certain 
applications the laser output may be transmitted directly to a compatible 
detector without the aid of an optical fiber. 
A schematic diagram of a fiber optic hydrophone that uses the laser 
modulation device is shown in FIG. 9. The laser 20 is coupled via a 
monomode optical fiber 39 to a sensor cavity 50, formed by inserting two 
partial reflectors 51 and 52 in the monomode fiber. Light emerging from 
the sensor cavity 50 is guided via an optical fiber 53 to detector 54. The 
detector provides an electrical input signal to a feedback circuit 55 
which drives the laser modulating transducer 56. 
In operation, the optical length of the laser cavity is maintained equal to 
a submultiple of the optical length of the sensing cavity by the voltage 
output from the feedback circuit, details of which will be given later. 
When such a condition is satisfied, the longitudinal modes of the laser 
are matched to maxima in the spectral response of the sensor cavity, so 
that the detected intensity is maximum. If the length of the sensor cavity 
varies, due to an acoustic pressure wave, a control voltage applied to the 
transducer re-establishes the coincidence of the response peaks. 
FIG. 10 gives a possible implementation of the feedback circuit. The 
control voltage applied to the transducer is the sum of three signals: (i) 
a high frequency (several kilohertz) signal which generates a fluctuation 
of the laser modes about the maxima of the spectral response of the sensor 
cavity; (ii) a correction signal which re-centers the oscillating modes on 
the response peaks at each cycle; (iii) the retrieved signal, obtained by 
integrating over time the correction signal, which represents the pressure 
fluctuations at the sensor cavity. 
The maximum modulating frequency that can be used in the implementations 
described above is limited by the sound velocity in the piezoelectric 
crystals and/or the laser chip. If modulation is to be achieved by 
longitudinal strain of the chip the chip can be no more than one-half 
wavelength long. For a chip of 0.5 mm length and a sound velocity of 4000 
m/sec, the maximum modulating frequency is 4 MHz. In some communications 
systems this may be restrictive, although not for the proposed hydrophone 
application, and it is probable that much higher modulating frequencies 
can be achieved by subjecting the laser diode to a transverse pressure 
wave. Then the limiting dimension would be the lasing region of the chip 
which is about 5 .mu.m wide and 1 .mu.m thick. In this case the principal 
modulating mechanism would be the refractive index change in the aluminum 
gallium arsenide due to the pressure wave, rather than the length change 
of the laser. 
From the foregoing it will be apparent that a new and useful method of 
varying the output of a solid state diode laser has been described. The 
output which is modulated in synchronism with a controllable modulating 
source may be used to communicate information via line of sight 
communication systems or through an optical fiber link.