System for controlling the wavelength and colinearity of multiplexed laser beams

• US 4,847,479 A
• Filed: 06/06/1988
• Issued: 07/11/1989
• Est. Priority Date: 06/06/1988
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Corporation
• Original Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Inventors: Heflinger, Lee O., Clark, George L.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Oen, William L.
• Application Number:
  US07/202,358
• Time in Patent Office: 400 Days
• Field of Search: 250/201 R, 250/201 AF, 250/205, 250/550, 372/35, 356/345, 356/346, 356/349, 356/400, 356/153, 219/121.78, 219/121.79, 219/121.80, 219/121.81, 219/121.83
• US Class Current: 250/201.1
• CPC Class Codes: H01S 5/005 Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping H01S5/026, H01S5/18388 take precedence
• CPC Class Codes: H01S 5/06243 controlling the position or direction of the emitted beam
• CPC Class Codes: H01S 5/06837 Stabilising otherwise than by an applied electric field or current, e.g. by controlling the temperature
• CPC Class Codes: H01S 5/0687 Stabilising the frequency of the laser
• CPC Class Codes: H01S 5/4012 Beam combining, e.g. by the use of fibres, gratings, polarisers, prisms
• CPC Class Codes: H01S 5/4025 Array arrangements, e.g. constituted by discrete laser diodes or laser bar H01S5/42 takes precedence
• CPC Class Codes: H01S 5/4087 emitting more than one wavelength

Abstract

Apparatus and a related method for control of the colinearity of multiple laser output beams in a wavelength-multiplexed optical communications system. Multiple lasers of different wavelengths formed into a single composite beam by means of a diffraction grating and other optical components are subject to loss in colinearity due to wavelength drift and mechanical alignment drift. The apparatus of one disclosed embodiment of the invention includes an alignment detector for the output beam and a set of tracer modulation detectors responsive to tracer modulation signals impressed on the individual laser beams. Misalignment detected in any one or more of the laser beams contained in the output beam is corrected by temperature control of the appropriate lasers. In another disclosed embodiment of the invention, misalignment is detected by partially reflecting the output beams back through the diffraction grating and other optical components to the lasers, but at angles slightly divergent from the common plane of the lasers. Position detectors located adjacent to the lasers, but not in the same plane, receive the return beams and are coupled to temperature control devices to adjust the individual lasers'"'"' wavelength, and hence maintain the colinearity of the output beams.

Claims

1. Apparatus for maintaining colinearity of multiple output beams in a wavelength multiplexer of diode lasers of different wavelengths, the apparatus comprising:

means for splitting off a sample beam from the combined output beams;

means for identifying the separate beams;

means for detecting any misalignment in each of the separate output beams, and generating control signals indicative of the degree of misalignment; and

means responsive to the control signals, for controlling the temperature of the individual diode lasers and thereby bringing the separate output beams back into alignment.

2. Apparatus as defined in claim 1, wherein:

the means for splitting off the sample beam includes a partially reflective mirror positioned in the path of the output beams.

3. Apparatus as defined in claim 2, wherein the means for detecting any misalignment includes:

focusing means;

split detector means having a reflective prism onto which the sample beam is focused, to produce two identical but oppositely directed beams if there is perfect alignment, and a pair of photo-detectors positioned to receive the oppositely directed beams; and

a difference amplifier coupled to receive the outputs of the photodetectors, for producing difference signals indicative of the misalignment of the beams.

4. Apparatus as defined in claim 3, wherein the means for identifying separate beams includes:

a tracer modulation generator for generating unique tracer signals for application to the separate laser beams;

means for modulating the laser beams with the tracer signals; and

means for detecting the separate tracer signals in the output signals from the difference amplifier, and thereby providing control signals for each of the means for controlling the temperature of the lasers.

5. Apparatus as defined in claim 4, wherein:

the means for controlling the temperature include coolers associated with respective ones of the lasers.

6. Apparatus for maintaining colinearity of multiple output beams in a wavelength multiplexer of diode lasers of different wavelengths, the apparatus comprising:

a plurality of laser diodes operating at different wavelengths;

an optical wavelength multiplexer operative to produce colinear output beams from multiple input beams of different wavelengths impinging at different angles but lying in common wavelength plane;

focusing means for directing multiple laser beams from the diodes onto the optical wavelength multiplexer to produce colinear output beams;

a partially reflective mirror positioned in the path of the output beams and oriented to provide return beams reflected back through the multiplexer and the focusing means at an angle slightly divergent from the wavelength plane;

a plurality of position detectors located one at each laser diode to receive the respective return beams and thereby detect any drift in wavelength; and

means responsive to the position detectors, for controlling the temperatures of the individual diode lasers and maintaining the output beams in alignment.

7. A method for compensating for wavelength and colinearity errors in a wavelength-multiplexed communications system, the method comprising the steps of:

modulating each a plurality of different-wavelength laser beams with a unique tracer signal;

splitting off a sample beam from a combined output beam that includes multiple beams of different wavelengths;

detecting any misalignment in the sample beam;

generating difference signals indicative of any detected misalignment;

identifying tracer signals in the difference signals and thereby associating the difference signals with one or more lasers; and

applying the difference signals to respective temperature control devices to adjust the temperature and hence the wavelength of the lasers to bring the output beams into colinearity.

8. A method for compensating for wavelength and colinearity errors in a wavelength-multiplexed communications system, the method comprising the steps of:

directing multiple laser beams of different wavelengths from coplanar laser diodes into an optical wavelength multiplexer capable of producing generally colinear output beams from appropriately directed input beams of different wavelengths lying in the same plane;

reflecting a portion of the output beams back through the multiplexer as return beams oriented at angles slightly divergent from the plane of the input beams;

detecting the return beams with detectors located adjacent to but not coplanar with the laser diodes; and

controlling the temperatures of the individual laser diodes in response to the detected return beams, to adjust the wavelengths of the diodes and hence maintain colinearity of the output beams.