Laser Apparatus

This invention relates to laser apparatus, and more particularly to laser apparatus of the type which can successively sweep the frequency of the laser's output beam over a wide range.

There is frequently a requirement in optical communications, measurement and other fields for a laser apparatus the output beam of which can be swept in frequency over a wide range. A known laser apparatus which can sweep the frequency of its output beam comprises a laser device such as a semiconductor, dye, or solid state laser and a pair of optical resonators having wavelength selectivity. The frequency of such a laser apparatus is swept by successively changing the angle of the optical resonators. Another type of known laser apparatus has a non-linear crystal. The frequency of this apparatus then may be swept by changing the temperature of the non-linear crystal. These known laser apparatuses, however, are bulky and cannot sweep the frequency quickly enough.

Semiconductor lasers which can rapidly sweep their frequency bands are known. Such a laser is described in a paper entitled "Integrated Electro-Optic Intracavity Frequency Modulation of Double-Heterostructure Injection Laser" by F. K. Reinhart et al, Applied Physics Letter, Vol. 27, No. 10, Nov. 15, 1975. However, this type of laser apparatus is limited in the range of frequencies over which it can sweep.

It is therefore an object of this invention to provide a laser apparatus which can sweep the frequency of its output beam over a wide range.

It is another object of this invention to provide a laser apparatus which can rapidly sweep its frequency.

It is yet another object of this invention to provide a relatively small-sized laser apparatus.

Specifically, the present invention provides a laser apparatus for use in the field of communications. It includes a first laser device able to sweep a first frequency band, and a second laser device able to sweep a second frequency band different from the first frequency band, portions of the first and second bands overlapping each other. The invention also includes a multiplexer for multiplexing the laser beams emanating from the first and second laser devices. A photodetector which detects the difference between frequencies of the first and second laser devices receives the output of the multiplexer. Control means connected to the photodetector cause the laser devices sequentially and alternately to sweep their respective frequency bands.

Other objects and features of this invention will become apparent to those skilled in the art from the following detailed description of preferred embodiments of the invention and the claims, as illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic view of a preferred embodiment of the laser apparatus according to the present invention;

FIG. 2 is a sectional view of the laser devices shown in FIG. 1;

FIG. 3 is a graphical representation of the frequency sweeping ranges of the laser devices shown in FIG. 1;

FIGS. 4A to 4D are graphical representations of various waveforms explaining the operation of the laser apparatus shown in FIG. 1;

FIG. 5 is a graphical representation of frequency sweep output beam with respect to time illustrating the operation of the laser apparatus shown in FIG. 1;

FIG. 6 is a schematic view of an alternative embodiment of the present invention; and

FIG. 7 is a graphical representation of various waveforms explaining the laser apparatus shown in FIG. 6.

FIG. 1 shows a schematic view of a laser apparatus of the present invention. This laser apparatus includes a first laser device 20a and a second laser device 20b. These laser devices may be any laser device capable of altering its frequency in response to an electrical signal. In the preferred embodiment first and second laser devices 20a and 20b are selected to be semiconductor lasers of the type described in the article by F. K. Reinhart referred to earlier. First laser device 20a is designed to have a sweep frequency band from a lower frequency f.sub.10 to upper frequency f.sub.11. Second laser device 20b is designed to have a sweep frequency band from a lower frequency f.sub.20 (less than f.sub.11) to an upper frequency f.sub.21 (greater than f.sub.11). The sweep frequency range of second laser device 20b is different from that of first laser device 20a except for an overlapping range of frequencies f.sub.20 -f.sub.11.

Laser output beams 22a and 22b emanate from first output faces of laser devices 20a and 20b respectively, and are transmitted to a first multiplexer 26 through optical gate switches 24a and 24b, respectively. First multiplexer 26 multiplexes laser output beams 22a and 22b and transmits the resulting laser beam 28 to an output port thereof.

Laser output beams 30a and 30b emanate from second output faces of laser devices 20a and 20b, respectively, and are transmitted to a second multiplexer 32 for multiplexing the laser output beams. The output beam of second multiplexer 32 is transmitted to a photodetector 34, which may be a photodiode.

Photodetector 34 senses the difference between the frequencies of the components of output laser beams 30a and 30b of first and second laser devices 20a and 20b in the output of multiplexer 32 and generates an output electric signal having a frequency related to the differential frequency.

The output signal of photodetector 34 is coupled to a frequency monitor 36 such as a frequency counter or a combination circuit including a low-pass filter and a level detector. Frequency monitor 36 continuously monitors the frequency of the output signal from photodetector 34. Frequency monitor 36 generates a control signal when the frequency of the output from photodetector 34 approaches zero and applies the control signal to a control circuit 38.

Control circuit 38 comprises a T-type flip-flop circuit 40 whose output state is inverted whenever an input pulse is received, a first voltage circuit 42a, and a second voltage circuit 42b. The input terminal of flip-flop circuit 40 is connected to the output terminal of frequency monitor 36 and the output terminals F and F of flip-flop circuit 40 are connected to first and second voltage sweep circuits 42a and 42b, respectively.

First voltage sweep circuit 42a includes a first transistor 46a having its base connected to the output terminal F of flip-flop circuit 40 and its collector connected to a constant current source 44a. A second transistor 48a has its base connected to the output terminal F of flip-flop circuit 40, its collector connected to the emitter electrode of first transistor 46a, and its emitter electrode connected to the ground. A capacitor 50a is connected between the emitter electrode of first transistor 46a and ground. The collector of transistor 48a and the emitter of transistor 46a are also connected to the base of transistor 52a, which is used in an emitter follower configuration. Accordingly, the collector of transistor 52a is connected to voltage source V.sub.2, and the emitter is connected to ground through resistor 55a. The output 54a, taken from the emitter of transistor 52a is applied as a sweep control signal to first semiconductor laser 20a.

Second voltage sweep circuit 42b has the same configuration as first voltage sweep circuit 40a except that the bases of a first transistor 46b and a second transistor 48b are connected to the outputs F and F of flip-flop circuit 40 respectively, and the output 54b of transistor 52b is applied as a sweep control signal to second semiconductor laser 20b.

The outputs F and F of flip-flop circuit 40 are also coupled as switching control signals to optical gate switches 24a and 24b, respectively. Thus, optical gate switches 24a and 24b are controlled so that when one optical gate switch is "ON", the other optical gate switch is "OFF".

Referring now to FIG. 2, there is shown a sectional view of the semiconductor laser devices used in the presently preferred embodiment. These laser devices include an n-GaAs substrate 200, an n-GaAlAs cladding layer 202 disposed on substrate 200, an optical waveguide layer 204 disposed on cladding layer 202, an n-GaAs tapered active waveguide layer 206 disposed on optical waveguide layer 204, a P-GaAlAs cladding layer 208 disposed on active waveguide layer 206, and a P-GaAlAs cladding layer 210 disposed on optical waveguide layer 204. Namely, the laser devices have double-heterostructure. Laser oscillation may take place by supplying a current I to the P-N junction between cladding layer 208 and active waveguide layer 206. The oscillation frequency of the laser device may be swept by successively changing a control voltage supplied between cladding layer 210 and substrate 200 to change the refractive index of optical waveguide layer 204. The operation of the embodiment having the above construction will now be described.

Referring now to FIG. 3, let it be assumed that first laser device 20a can, in response to a voltage signal, vary the frequency f.sub.1x of its output beam 22a between f.sub.10 and f.sub.11, and that second laser device 20b can similarly vary the frequency f.sub.2x of its beam 22b between f.sub.20 and f.sub.21. Let it be further assumed that f.sub.1x =f.sub.10 and f.sub.2x =f.sub.20 and optical gate switch 24a is "ON" and optical gate switch 24b is "OFF" at the initial state. In this state only the output laser beam 22a from first laser device 20a reaches first multiplexer 26. On the other hand, output laser beams 30a and 30b are both multiplexed by second multiplexer 32. The multiplexed laser beam is transmitted to photodetector 34. Photodetector 34 detects the differential frequency signal component in this case .DELTA.f=f.sub.20 -f.sub.10, between the output laser beams of first and second laser devices 20a and 20b in the multiplexed laser beam.

Initially, flip-flop circuit 40 in control circuit 38 is set with output F at logic "1" and output F at logic "0", as shown in FIGS. 4A and 4B. Under these conditions, first transistor 46a in voltage sweep circuit 42a is operative and second transistor 46b is not operative. In the operation of first transistor 46a the current I from constant current source 44a flows into capacitor 50a through first transistor 46a and an electric charge is accumulated in it. This causes the output voltage 54a of emitter follower circuit 52a, which serves as sweep control voltage for first laser device 20a, to rise gradually as shown in FIG. 4C and first laser device 20a responds by sweeping its characteristic frequency band. The oscillation frequency f.sub.1x of first laser device 20a thus rises gradually from the lowest frequency f.sub.10 to the high frequency f.sub.11.

During this entire process, frequency f.sub.2x of second laser device 20b remains constant (f.sub.2x =f.sub.20). Therefore, the output frequency of photodetector 34 may be expressed as follows:

This differential frequency .DELTA.f decreases as the frequency f.sub.1x increases. As .DELTA.f approaches zero, frequency monitor 36 delivers a control signal to flip-flop circuit 40 in control circuit 38. This control signal inverts the output state of flip-flop circuit 40 as shown in FIGS. 4A and 4B. Thus, the output F inverts to logic "0" and the output F inverts to logic "1". In voltage sweep circuit 42a, first transistor 46a cuts off and second transistor 48a turns on, and the electric charge accumulated in capacitor 50a is discharged instantly to the ground through second transistor 48a. The output of transistor 52a as the sweep control signal for first laser device 20a reduces as shown in FIG. 4C. Accordingly, first laser device 20a ceases sweeping of its frequency band and assumes the value f.sub.10.

Meanwhile in voltage sweep circuit 42b, first transistor 46b connected to the output F of flip-flop circuit 40 turns on and second transistor 48b connected to the output F of flip-flop circuit 40 cuts off. The current I from constant current source 44b then flows into capacitor 50b through first transistor 46b and an electric charge is accumulated in it. The output voltage 54b of transistor 52b rises gradually as shown in FIG. 4D and second laser device 20b begins to sweep its frequency band. At the same time optical gate switch 24b switches "ON" and optical gate switch 24a is changed to "OFF" by the inversion of the output of flip-flop circuit 40. Therefore, in this state the output laser beam 22b of second laser device 20b reaches first multiplexer 26, while output laser beam 22a does not.

The frequency f.sub.2x of second laser device 20b is swept from its lowest frequency f.sub.20 to its highest frequency f.sub.21. Accordingly, the frequency of the laser apparatus according to this embodiment may be swept successively from the frequency f.sub.10 which is the lowest frequency of first laser device 20a to the frequency f.sub.21 which is the highest frequency of second laser device 20b.

When the frequency of second laser device 20b reaches f.sub.21, a control signal is generated for resetting flip-flop 40. This signal is generated as follows. The voltage at the emitter of transistor 52b, which causes laser 20b to sweep, is compared in a comparator 80 with a reference signal Vs. A comparison signal is generated when this emitter voltage becomes greater than Vs. This comparison signal is coupled to the reset input of flip-flop 40. This control signal inverts the output state of flip-flop circuit 40. Accordingly, second laser device 20b ceases sweeping of its frequency band and first laser device begins to sweep its frequency band again. The operation mentioned above is carried out repeatedly and the first and second frequency bands are swept sequentially and alternately as shown in FIG. 5.

As described above, a laser apparatus according to the present invention can sweep the frequency of the laser oscillation signal over a wide range and can sweep it quickly because the output frequency of the laser devices 20a and 20b may be swept by changing the sweep control voltage. Furthermore, the laser apparatus of the present invention has a relatively simple and compact construction.

FIG. 6 illustrates another embodiment of the present invention in which a multiplicity of laser devices is used to expand the sweep frequency range.

The sweep frequency ranges of semiconductor lasers 20.sub.1, 20.sub.2, . . . , and 20.sub.n are different respectively except for a part of overlapped ranges. That is, the maximum frequency of semiconductor laser 20.sub.1 marginally exceeds the minimum frequency of semiconductor laser 20.sub.2, whose maximum frequency in turn just barely exceeds the minimum frequency of semiconductor laser 20.sub.3. In general, the maximum frequency of semiconductor laser 20.sub.n-1 will just barely exceed the minimum frequency of semiconductor laser 20.sub.n.

Laser output beams 22.sub.1, 22.sub.2, . . . , and 22.sub.n emerge from each first output face of semiconductor laser 20.sub.1, through the optical gate switches 24.sub.1, 24.sub.2 . . . , and 24.sub.n, respectively.

Laser output beams 30.sub.1, 30.sub.2, . . . , and 30.sub.n emerge from each second output face of semiconductor lasers 20.sub.1, 20.sub.2 . . . , and 20.sub.n are transmitted to the second multiplexer 32 through optical gate switches 60.sub.1, 60.sub.2 . . . , and 60.sub.n respectively. The output laser beam from second multiplexer 32 is converted into an electric signal by the photodetector 34. The output of photodetector 34 is delivered to the frequency monitor 36. The output of frequency monitor 36 is delivered to a control circuit 380 which comprises the voltage sweep circuits 42.sub.1, 42.sub.2, . . . and 42.sub.n as shown in FIG. 1, a first shift register 62 for controlling the operation of voltage sweep circuits 42.sub.1, 42.sub.2 . . . , and 42.sub.n and first optical gate switches 24.sub.1, 24.sub.2, . . . , and 24.sub.n, and a second shift register 64 for controlling the operation of second optical gate switches 60.sub.1, 60.sub.2, . . . , and 60.sub.n. First shift register 62 has a first input terminal 66 responsive to the output of frequency monitor 36 and a second input terminal 68 to which a logic signal "1" is applied at the beginning of a sweep operation. The logic signal "1" in the shift register 62 is shifted one bit whenever an output pulse has been received from frequency monitor 36. The output of each stage of first shift register 62 is applied to one of voltage sweep circuits 42.sub.1, 42.sub.2, . . . , and 42.sub.n and first optical gate switches 24.sub.1, 24.sub.2, . . . , and 24.sub.n, respectively. Therefore, exactly one of the voltage sweep circuits and its associated first optical gate switch are always operational.

Second shift register 64 has a first input terminal 70 connected to the output from frequency monitor 36 and a second input terminal 72 through which a pair of logic signals "1" are input into second shift register 64 at the beginning of the sweep operation. The pair of logic signals "1" are shifted one bit upon receipt of output from frequency monitor 36. The output of each stage of second shift register 64 is applied to second optical gate switches 60.sub.1, 60.sub.2, . . . , and 60.sub.n, respectively. Therefore, an adjacent pair of second optical gate switches is always "ON".

The operation of the embodiment having the above-mentioned construction will now be described with reference to FIG. 7. In FIG. 7, P1, P2, . . . , and Pn indicate whether first optical gate switches 24.sub.1, 24.sub.2, . . . , and 24.sub.n are "ON" or "OFF". Similarly, Q1, Q2, . . . , and Qn indicate whether second optical gate switches 60.sub.1, 60.sub.2, . . . , and 60.sub.n are "ON" or "OFF".

As an initial condition, each semiconductor laser 20.sub.1, 20.sub.2, . . . , and 20.sub.n oscillates at its lowest frequency f.sub.10, f.sub.20, . . . , and f.sub.no, optical gate switch 24.sub.1 is "ON", optical gate switches 24.sub.1, 24.sub.2, . . . , and 24.sub.n are "OFF", and optical gate switches 60.sub.1 and 60.sub.2 are "ON" and optical gate switches 60.sub.1, 60.sub.2, . . . , and 60.sub.n are "OFF".

As the oscillation frequency of semiconductor laser 20.sub.1 gradually increases in response to an increasing voltage signal from voltage sweep circuit 42.sub.1, an output signal having a frequency representing the difference between frequencies f.sub.1x and f.sub.20 of semiconductor lasers 20.sub.1 and 20.sub.2 appears at photodetector 34. As f.sub.1x approaches very close to f.sub.20, frequency monitor 36 applies a control signal to control circuit 380. Control circuit 380 turns optical gate switches 24.sub.1 and 60.sub.1 "OFF" and turns optical gate switches 24.sub.2 and 60.sub.3 "ON". Control circuit 380 also halts the sweep operation of semiconductor laser 20.sub.1 and initiates the sweep operation of semiconductor laser 20.sub.2. Then, as f.sub.2x approaches very close to f.sub.30, optical gate switches 24.sub.1 and 60.sub.2 are turned "OFF", optical gate switches 24.sub.3 and 60.sub.4 are turned "ON" and semiconductor laser 20.sub.3 starts sweeping the oscillation frequency in place of semiconductor laser 20.sub.2. These "hand-offs" occur repeatedly until the semiconductor laser 20.sub.n having the highest maximum frequency has completed its sweep. Thereby, the oscillation frequency of laser apparatus according to this embodiment may be swept successively from the frequency f.sub.10 which is the lowest oscillation frequency of semiconductor 20.sub.1 to the frequency f.sub.n1 which is the highest oscillation frequency of semiconductor laser 20.sub.n.

As alternatives to the embodiments shown in FIGS. 1 and 6, the semiconductor laser devices 20a, 20b, 20.sub.1, 20.sub.2, . . . 20.sub.n may be replaced by another laser device capable of sweeping a frequency bandwidth in response to a voltage control signal. Also, the number of laser devices 20.sub.1, 20.sub.2, . . . , and 20.sub.n shown in FIG. 6 may be selected with reference to the desired sweep oscillation range and a sweep oscillation range of the individual laser devices. Second optical gate switches 60.sub.1, 60.sub.2, . . . , and 60.sub.n shown in FIG. 6 need not be employed if adjacent pairs of conductor laser devices are driven sequentially. The sweep of the oscillation frequency of the laser devices may also progress in reverse from the highest oscillation frequency toward the lowest oscillation frequency. The control circuits 38 and 380 are not limited to the embodiments shown in FIGS. 1 and 6.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures.