Multimode stabilized external cavity laser

An etalon stabilized external cavity laser. The present invention is embodied in an external cavity laser and utilizes a servo circuit to adjust the angle of an intra-cavity etalon to suppress multimode operation of the laser. The servo circuit employs a diode detector to detect a beat frequency that occurs during multimode laser operation. The output from the diode is amplified, compared and integrated to produce a control signal. This signal is used to control the angle of the etalon and thereby suppress the multimode operation of the laser.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to lasers and more particularly to etalon 
stabilized, grating tuned, external cavity semiconductor lasers. 
2. Description of the Prior Art 
Future optical communications systems employing heterodyne methods will 
require turnable, single-frequency, narrow-linewidth, semiconductor 
lasers. Such laser systems will be modulated and coupled to optical fiber 
cables for transmission of the light energy to a distant receiver. This 
type of laser optical system has distinct bandwidth advantages over the 
systems currently in use which makes it attractive for high-bit-rate and 
long-haul optical communications systems. 
For heterodyne communications and other coherent (single frequency) 
applications, such as heterodyne signal analysis, the line-width (optical 
bandwidth) of monolithic single frequency lasers may not be sufficiently 
narrow for acceptable system performance. External cavity, grating tuned, 
semiconductor lasers, however, have the narrow linewidth and tuning range 
needed for such applications. 
The narrow linewidth of the external cavity laser is particularly important 
in optical heterodyne spectrum analysis equipment. In these systems, the 
linewidth of the local oscillator used in such equipment should be much 
narrower than that of the signal under test. In addition, the longitudinal 
sidemode suppression ratio of the local oscillator must be very high, in 
excess of 30db, since any sidemodes on the local oscillator would be 
indistinguishable from spectral features on the signal under test. 
The design of grating tuned external cavity semiconductor lasers is well 
known in the art. An example of an external cavity laser design is 
disclosed in U.S. Pat. No. 4,942,583 issued to Paul Zorabedian et al. and 
assigned to Hewlett-Packard Company. 
One drawback of external cavity lasers is their tendency to hop (switch) 
between two or more longitudinal mode frequencies (multimode) during 
tuning or when the bias current is increased to maximize the laser output 
power. The multimode operation occurs because the bandwidth of the 
external cavity laser is not sufficiently narrow to exclude all but one 
frequency of operation. 
There are three general types of multimode operations. Although not always 
the case, the three types of multimode operations tend to occur at 
successively higher ranges of drive current as shown in FIG. 1. 
The type I multimode operation tends to occur nearest the threshold of the 
semiconductor laser. However, it can extend to the semiconductor's upper 
current limit. As shown in FIG. 2, the type I multimode condition is 
characterized by a strong main mode and two weak symmetric sidemodes. The 
sidemode ratio may be anywhere from approximately -10 to -30 db. This is 
the mildest form of laser multimode operation with the linewidth of the 
main mode comparable to that of a single oscillating mode (approximately 
50-100 kilohertz "KHz"). 
The type II multimode operation, as shown in FIG. 3, tends to occur at 
higher current levels than type I but it sometimes occurs right at the 
threshold current of the semiconductor. There may be several modes with 
approximately equal energy levels and there may be no discernable main 
mode. The linewidths of the individual modes may be somewhat broadened 
compared to type I and of approximately 100 KHz to 10 MHz. With the type 
II multimode condition, the laser energy may be spread over several 
frequencies and the laser system will no longer function in coherent 
applications. 
The type III multimode operation occurs at the highest current range of the 
semiconductor. The energy is spread more or less uniformly over many 
frequencies, as shown in FIG. 4, each of which is broadened to a GHz or 
more. The type III multimode operation is the regime of "coherence 
collapse". This collapse is generally due to the onset of optical chaos in 
the external cavity where no single main mode is amplified. 
Attempts have been made to control external cavity lasers to prevent 
multimode operation. M. Ohtsu et al describe one such attempt in the 
"Journal of Lightwave Technology", Vol. 7, No. 1, Jan. 1989, in the 
article titled "A Simple Interferometric Method for Monitoring Mode 
Hopping in Tunable External-Cavity Semiconductor Lasers". Their 
experimental apparatus is shown in FIG. 5. 
In the Ohtsu et al experiment, an automatic control was built using a 
electro-optical servo loop to suppress multimoding. A fiber delay line 501 
was used to increase the temporal overlap of the laser modes thereby 
increasing the resultant optical signal. A local oscillator 503 and mixer 
505 were used to down convert the beat signal from the detector 507, 
typically a few GHZ, to a few hundred MHZ. The down conversion process 
produces a radio frequency (RF) signal that is used to control a piezo 
electric transducer (PZT) 509. 
Ohtsu et al. uses the presence of the mode beat signal to change the cavity 
length. This is accomplished by varying the voltage to the PZT and thereby 
moving the grating 511. The movement of the grating does not change the 
pass bandwidth of the optical feedback, only the feedback phase. If the 
change in the grating position causes the laser output to become single 
mode, the beat signal disappears and the present voltage to the PZT is 
maintained thereby maintaining the feedback phase at the proper value. 
The inventor's experimental results have shown that adjusting the length of 
the cavity does not suppress all three types of multimode operation. 
Although changing the length of the cavity suppresses type I multimode 
operation, it is only partially effective in suppressing type II multimode 
operation and it is ineffective in suppressing type III multimode 
operation. Additionally, the cavity length sometimes requires readjustment 
if the laser current is changed even within one particular regime of 
multimode operation for which it is effective. 
In the "Journal of Lightwave Technology", Vol. LT-5, No. 4, April 1987, 
N.A. Olsson et al published an article titled "Performance Characteristics 
of 1.5 um External Cavity Semiconductor Lasers for Coherent Optical 
Communication". This article describes an external cavity laser system 
that uses feedback to control the cavity length with a PZT. Also described 
is an intra-cavity etalon that narrows the passband of the cavity. The 
angle of the etalon is changed to tune between the internal modes of the 
laser. As is well known in the art, the etalon will periodically narrow 
the optical passband width of the grating as the etalon is increasingly 
tilted in the intracavity beam. The Olsson et al system is shown in FIGS. 
6 and 7. 
The arrangement of the basic elements of the Olsson et al external cavity 
laser is best shown in FIG. 6. The light from a laser diode 601 is 
collimated by a lens 603 and projected through an etalon 605 and 
retroreflected back to the laser by a grating reflector 607. A PZT element 
609 is mechanically coupled to the reflector 607 and is used to adjust the 
optical path length. 
To stabilize the laser, a feedback system is provided as shown in FIG. 7. 
The feedback system uses a beat detector 701, a mixer 703 to convert the 
detector output to a lower frequency, a low pass filter 705 to convert the 
mixer output to a DC (direct current) control level and a high voltage 
amplifier 707 to amplify the DC control level to the high voltages 
required to drive the PZT (not shown). 
Tuning of the Olsson et al external cavity laser is divided into coarse, 
medium, and fine tuning. Coarse tuning is performed by rotating the 
grating reflector 607 and selecting the internal mode which is closest to 
the desired wavelength. This provides a tuning range of approximately 900 
A (Angstroms). The internal modes refer to the longitudinal modes of the 
laser diode without the external cavity. These modes appear because of the 
imperfect anti-reflection coating on the laser facet. Medium tuning is the 
tuning of the laser in between its internal modes, a range of appoximately 
19 A. This tuning is achieved by manually adjusting the intra cavity 
etalon 605 in combination with a fine rotation of the grating reflector 
607. Fine tuning is performed by adjustments to the external cavity length 
that varies the laser frequency within the megahertz range. The PZT 609 is 
connected to the reflector 607 to facilitate this adjustment. 
While the Olsson et al external cavity laser system may eliminate all three 
types of multimode operations with multiple manual adjustments, there 
still exists the need for a laser system that eliminates all three types 
of multimode operations automatically. 
SUMMARY OF THE INVENTION 
The external cavity laser of the present invention automatically eliminates 
all three types of multimode operations and can therefore be used to great 
advantage in optical communications and coherent measurement applications. 
Furthermore, the present invention achieves the elimination of multimode 
operations by the adjustment of a single element instead of multiple 
elements required by prior art devices thereby simplifying the design of 
the laser system. 
The present invention uses a semiconductor laser to generate a beam of 
coherent light. An etalon is mounted on a galvanometer and is located 
within the optical path between the semiconductor laser and a 
retroreflecting grating such that the beam passes through the etalon and 
is reflected back to the laser semiconductor by the grating. 
A feedback circuit is used to control the galvanometer and thereby control 
the angle of the etalon. The feedback circuit works by converting an 
output from a multimode (beat frequency) detector into a control current 
that is used to drive the galvonometer. As the galvanometer moves, the 
etalon angle, relative to the laser beam, changes until multimode 
operation is eliminated. 
Because only one element of the external cavity laser is adjusted, the 
design, manufacture and use of the laser system is greatly simplified.

DESCRIPTION OF THE INVENTION 
As is shown in FIG. 8, the present invention is embodied in a tunable 
external cavity laser. A semiconductor laser 801 generates a first laser 
beam along a path 803 and a second laser beam along a path 805. The first 
laser beam is focused by a lens 806 and passes through a fiber optic 
splitter 807 which splits the beam into two parts. One part is coupled to 
a beat detector 809 and the other part becomes the laser output 811. 
The second laser beam passes through a collimating lens 813 that focuses 
the beam through an etalon 815 and onto a retroreflector grating 817. A 
galvanometer 819 is mechanically coupled to, and is able to rotate, the 
etalon 815. The etalon is approximately 5 mm thick and has a reflective 
coating of approximately R=4%, per surface, at 1300 nm (nano-meters). 
A feedback system 821 is provided to adjust the etalon 815. This feedback 
system includes the beat detector 809, such as an AT&T avalanche photo 
diode (APD), connected to an amplifier 823 such as an Avantek AMG 4046M. 
The output from the amplifier 823 is connected to an envelope detection 
circuit 825 that is in turn connected to an integrator 827. A voltage to 
current converter 829, such as an ILX Lightwave LDX-3207, is fed by the 
integrator 827 and the output from the converter 829 is connected to the 
galvanometer 819. 
If the laser operates in multiple modes, the different frequencies mix and 
the beat frequency is detected by the beat detector diode 809. The output 
from the diode 809 is typically a microwave signal that is amplified by 
the amplifier 823. The envelope detector 825 performs the function of a 
threshold detector. When the detector 825 receives a signal from the 
amplifier 823 that exceeds a preset level, the detector 825 outputs a DC 
voltage to the integrator 827. The output from the detector 825 has two 
levels, on or off. 
The integrator 827 generates an output voltage ramp that is proportional to 
the time the detector 825 outputs the DC voltage. As the time increases, 
the output voltage generated by the integrator 827 increases. This output 
voltage is converted to a current by the voltage to current converter 829. 
A higher voltage is converted to a higher current. 
A General Scanning G120D is used as the galvanometer 819 and the rotation 
of the galvanometer 819 is controlled by the current from the voltage to 
current converter 829. As the galvanometer rotates, the etalon 815 
rotates. 
At some rotation angle of the etalon 815, the pass band of the cavity will 
be narrowed sufficiently to suppress multimode operation. The beat 
frequency will disappear, and a signal will no longer be present at the 
input of the envelope detector 825. Without an input signal, the output 
from the envelope detector 825 switches to the off state and the output 
voltage from the integrater 827 is held by the integrator 827 at the 
current value. With a constant input voltage, the output from the voltage 
to current converter 829 remains constant thereby maintaining the angle of 
the etalon 805 constant. 
To tune the laser system to a different frequency, the retroreflector 
grating 817 is rotated to the correct angle, as is well known in the art. 
Also, the integrator 827 is reset causing the output voltage from the 
integrator 827 to go to the minimum value that in turn causes the etalon 
815 to rotate to a minimum angle. The integrator 827 is released from the 
reset state and the feedback system 821 now stablilizes the laser system 
at the new frequency. 
Of course, one skilled in the art will be able to modify the feedback 
circuit without departing from the present invention. For example, the 
integrator 827, as disclosed, holds the output voltage constant when the 
beat signal amplitude is reduced. One obvious change would be for the 
integrator 827 not to hold the output voltage. This change would cause the 
feedback circuit to continuously adjust the angle of the etalon 815 to 
reduce multimode operation. 
While the etalon has been described as having a reflectivity of 
approximately 4%, one skilled in the art would be able to adapt an 
uncoated etalon or an etalon with a higher reflectivity to work in this 
application. 
While a preferred embodiment of the present invention has be described 
above, it is intended that all matter contained in the above description 
and shown in the accompanying drawings be interpreted as illustrative and 
not in a limiting sense and all modifications, constructions and 
arrangements which fall within the scope of the invention be determined 
solely from the following claims.