Wavelength stabilization

WAVELENGTH STABILIZATION field cf the Invention The present invention relates to the stabilization of the wavelength of an optical signal emitted by an optical source such as a laser.

BaeXoround of the Invention Optical sources, particularly broadband optical sources such as semiconductor light sources, are used in a variety of applications in which a stable wavelength of the emitted optical signal is important. For example, in ring laser and fiber optic gyroscope systems, a light source emits a light beam which is split. The resulting two light beams are then sypplied to respective ends of an optical path. The two beams counterpropagate along the optical path, are recombined at the beam splitter and are received by a detection system for detecting the phase difference between the two counterpropagating light beams. If the gyroscope is at rest, the path length around the optical path is ideally the same for both light beams so that no phase difference between the two light beams wili be detected. However, as the gyroscope rotates, the time that is inquired for one beam to travel the path is different than the time that is required for the other beam to traverse the path. In effect, one of tha beam$ will travel a longer path than the other beam* Si«f the phases of the two beams are a function of the time required to travel their respective paths, any difference in that time between the two light beams will result in a phase difference.

- 2 Rotation of the gyroscope does not affect the frequency (i.e. wavelength) of the optical signal, However, environmental changes, such as temperature changes, can influence the wavelength of the signal supplied to the gyroscope by the optical source. Since the phase difference (i.e. phase shift) between the counterpropagating light beams is dependent upon wavelength as well as rotation, a constant: wavelength is necessary so that the relationship between rotation and phase difference is repeatable from measurement to measurement. Therefore, it is important to minimize the effects of these environmental changes on the wavelength of the source optical signal so that any changes in phase of the output optical signals is due to rotation and not to changes in environmental conditions.

• w • • O), Summary of the Invention The present invention provides an apparatus for providing a wavelength stabilized optical signal comprising:

source means for providing a source optical signal, said source optical signal having a source optical signal power and a wavelength; splitting means for receiving said source optical signal and for splitting said source optical signal into a first optical signal having a first optical signal power and a second optical signal having a second optical signal power so that said source optical signal power is split by said splitting means between said first optical signal power and said second optical signal power, said split between said first optical signal power and said second optical signal power being dependent upon said wavelength of said source optical signal ,- a first light detecting diode having an anode and cathode for receiving said first optical signal and for providing a first detector signal indicative of said first optical signal power; a second light detecting diode having an anode and a cathode for receiving said second optical signal and for providing a second detector signal indicative of said second optical signal power; and control meant, comprising connecting means for connecting the anode of said first light detecting diode to said cathode of said second light detecting diode so that the first and second detector signals are subtracted from one another, said control means providing an output signal representing the difference between said first detector signal and said second detector signal which is transmitted to said source means for controlling said wavelength of said source optical signal in response to said first and second detector signals.

In one preferred form of the invention, the anode of the first light detecting diode is connected to the positive input of a differential amplifier and the anode of the second light detecting diode may be connected to the negative input of the differential amplifier.

(Alternatively, the cathodes of the first and second light emitting diodes may be connected to the respective positive a,ind negative inputs of a differential amplifier.) The differential amplifier provides an output representative of the difference between the first optical signal power and the second optical signal power and is used for controlling the current supplied to the optical source to in turn control the wavelength of the source optical signal.

In another preferred form of the invention, the •;;• „ anode of one light detecting diode and the cathode of "• '« the other light detecting diode may be connected to a common node. Since the currents flowing between the node and the diodes will have opposite polarities, the output current from the node represents the difference of the currents of the two light detecting diodes and thus represents the difference between the first optical /S <7, signal power and the •» t • • I WO 93/11589 PCr/US92/10593 second optical signal power. The output current from the node is amplified if desired and used to control the current supplied to the optical source in order to control the wavelength of the source optical signal.

In yet another aspect of the invention, the first and second detectors may be used to control a temperature controller which in turn controls the temperature of the optical source. Since the wavelength of the optical source is temperature dependent, the control of the temperature of the optical source will control the wavelength of the source optical signal.

Brief Description of the Drawings These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which; Figure 1 shows a first embodiment of the wavelength stabilizing control system according to the present invention; Figure 2 shows a second embodiment of a wavelength stabilization control system according to the present invention; Figure 3 Shows the current driver of Figures 1 and 2 in more detail; and, Figure 4 shows a wavelength stabilization control system utilizing a temperature controller for controlling the wavelength of the source optical signal.

WO 83/11589 PCr/US92/l0593 Detailed Description Stabilization system 10, as shown in Figure 1, comprises an optical source, such as later 11, for emitting an optical signal 12, such as a light beam, having power P(A). Optical signal 12 enters port 13 of an optical component 14. Optical component 14 may be a WDM (wavelength division multiplexer), a beam splitter, an etalon, a grating, or the like. Optical component 14 splits the source optical signal 12 entering port 13 into a first optical signal (or light beam) 15 exiting port 16 and a second optical signal (or light beam) 17 exiting port 18.

The power contained in each of the optical signals 15 and 17 will be determined by the transmittance Ta) between port 13 and port 16 and the transmittance TjU) between port 13 and port 18 respectively. In the ideal case, Ta) and TjU) are complimentary land no transmission loss occurs. Thus, TA) + Tjt) is equal to 1. The transmittance T1(A) and TjtA) of these devices are, as indicated, dependent upon the wavelength a. Accordingly, the wavelength dependent output powers of optical signals and 17 are given by the following equations:

PA) - PS(A) * TA) (1) and P2(A) - PS(A) - T2(A) (2) where P{aJ represents the power of the source optical signal 12 emitted from optical source 11/ P1(A) represents the power of the first optical signal 15 exiting port 16 of ©ptieal component 14, P2(*) represents the power of the W093/U589 PCT/US92/10593 second optical signal 17 exiting port 18 of optical component 14, T1(A) represents the transmittance of optical component 14 between ports 13 and 16, TjfA) represents the transmittance of optical component 14 between ports 13 and IS, and the »* symbol represents multiplication.

The total power of the first optical signal which exits port 16 is given by the following equation:

Pi - Ix P1(Mdx (3) The total power of the second optical signal 17 which exits port 18 is given by the following equation:

Ideally all light which enters the component 14 exits ports 16 and 18. Since the transmittance of the exit ports is a function of wavelength of the source optical signal 12, a greater or lesser portion of the source optical signal 12 will transmit through exit port 16 rather than exit port 18 dependent upon the wavelength of the source optical signal.

Thus, as the wavelength of the source optical signal 12 changes, the split between P and Pj will change.

The power in the first optical signal 15 is sensed by detector 21 which may be a light detecting diode.

Similarly, the power in the second optical signal 17 is sensed by detector 22 which also may be a light detecting diode. The anode of light detecting diode 21 is connected to the positive input of differential amplifier 23 and the anode of light detecting diode of 52 is connected to the negative input of differential amplifier 23. Light detecting diode 21 produces an output current which is W093/n589 PCT/US92/10593 converted to a voltage by the differential amplifier 23 according to the following equation:

V1 = 14 * P1 (5) where V is the voltage produced by light detecting diode 21 and K is a constant which incorporates light detector efficiencies and electrical gains. Similarly, light detecting diode 22 produces an output current which is converted to a voltage by the differential amplifier 23 according to the following equation:

V2 = K2 * P2 (6) where V2 is the voltage produced by light detecting diode 22 and K, is a constant which incorporates light detector efficiencies and electrical gain. Differential amplifier 23 will provide an output signal representing the difference between v and Vj. This output signal is connected to voltage to current converter 24 which converts the output voltage from differential amplifier 23 to a current signal. This current signal is supplied to current driver 25. Current driver 2 5 responds to the current from voltage to current converter 24 so as to control the current supplied to optical source 11 to in turn control the wavelength of the source optical signal 12.

Light detecting diodes 21 and 22 may be selected so that constants K and K2 are equal. If so, current driver 25 is initially arranged to provide a current to optical source 11 such that the power of source optical signal 12 is evehly divided by optical component 14 between the power of first optical signal 15 and the power of second optical signal 17. When the power of the source WO 93/11589 PCr/US92/10593 op ical signal 12 is evenly divided between the powers of first and second optical signals 15 and 16, light detecting diodes 21 and 22 will supply equal voltages to the respective positive and negative inputs of differential amplifier 23. Thus, the output of differential amplifier 23 is 0 and no adjustment will be made to current driver 25. Qn the other hand, if optical source 11 experiences fluctuations of either the injection current or the environmental temperature, the wavelength of source optical signal 12 will change. This change in wavelength of source optical signal 12 will result in an unequal division of power between first optical signal 15 and second optical signal 17 which will result in unequal currents being supplied by light detecting diodes 21 and 2 2 to the inputs of differential amplifier 23. Accordingly, differential amplifier 23 will supply a non-zero output to voltage to current converter 24 which will convert that non-zero voltage to a current output. Current driver 25 will respond to this current from voltage to current ©onvertr 24 to adjust the injection current supplied to bptical source 11 to bring the wavelength of source optical signal 12 back to a value which produces a zero output from differential amplifier 23.

Alternatively, if light detecting diodes 21 and 22 are not matched (i.e. constants Kj and Kj are not equal), an injection current may be initially supplied to optical source 11 so that, although the power of source optical signal 12 is uhevenly split between first optical signal IS and second optical signal 11, the light detecting WO 93/11589 PCr/US92/10593 diodes 21 and 22 will supply equal voltages to differential amplifier 23, In this case, as long as the wavelength of the initial source optical signal 12 does not change, no adjustment to the injection current of optical source 11 will be mad. Hewever, if the wavelength of the source optical signal 12 changes for any reason, the output of differential amplifier 23 will become non-zero which will result in a current output from voltage to current converter 24. This current output will be used by current driver 25 to alter the injection current supplied to optical source 11 to bring the wavelength back to its initial value.

Instead of selecting an initial injection current (and, therefore, an initial source optical signal wavelength) to produce a zero voltage output fsom differential amplifier 23> voltage to current converter 24 can be initially arranged so that it supplies a zero output current driver 25 at the desired wavelength df optical signal 12 even though differential amplifier 23 supplies a non-zero voltage output. If the wavelength of the source optical signal 12 changes for any reason, the output of voltage to current converter 24 will become non-zero which will result in correction of the wavelength of source optical signal 12 until that wavelength is again at the desired value.

figure 2 shows an alternativ# which eliminates the need for a differential amplifier. In this case, the anode of light detecting diode 21 and the cathode of light detecttfig diode 22 are connected to node 27. With this WO 93/11589 PCT/LS92/10593 arrangement, the current flowing between light detecting diode 21 and node 2? will be of one polarity while the current flowing through light detecting diode 22 and node 27 will be of an opposite polarity. Thus, the current in line 28 represents the difference between these currents and, therefore, the difference in power between optical signals 15 and 17. This difference current may be amplified by an amplifier 26, if desired, and the resulting voltage supplied to voltage to current cojraVferter 24.

As shown in Figure 3, current driver 25 may include a summing junction 25' which will receive a current on input 29 equal to an initial injection current selected to provide the desired wavelength for optical signal 12.

The output from voltage to current converter 2 4 is connected to input 30 of summing junction 25'. Thus, any signal on input 30 acts as an error signal indicating that the wavelength of source optical signal 12 has drifted from its desired value. In this case, the error on input 3 0 will adjust the injection current in line 31 being supplied to optical source 11 to bring the wavelength of source optical signal 12 back to its desired value.

Alternatively, a desired wavelength may be selected which will produce an error signal at input 30 having a predetermined non-zero value. Thus, when this predetermined non-zero value is combined with the current at input 29, optical signal 12 will have the desired wavelength. However, any variation in wavelength of optical signal 12 will change the error signal away from its predetermined non-zero value, and thus change the WO 93/11589 PCT/US92/10593 current at output 31, until optical signal 12 regains its desired wavelength.

As shown in Figure 4, instead of controlling the injection current supplied to optical source 11, the temperature of optical source 11 may be controlled in order to in turn control the wavelength of source optical signal 12. In this case, voltage to current converter 24 and current drives* 25 shown in Figures 1 and 2 are replaced by temperature controller 40. Temperature controller receives the output from difference circuit 41 in order to adjust the temperature of optical source XI to control the wavelength of source optical signal 12. Difference circuit 41 may represent either differential amplifier 23 of Figure 1, node 27 of Figure 2, or any other arrangement for providing an output to temperature controller 4 0 based upon the difference in power between first and second optical signals 15 and 17.

• • -» a IS 2S ,r THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:- 1. An apparatus for providing a wavelength stabilized optical signal comprising:

source means for providing a source optical signal, said source optical signal having a source optical signal power and a wavelength; splitting means for receiving said source optical signal and for splitting said source optical signal into a first optical signal having a first optical signal power and a second optical signal having a second optical signal power so that said source optical signal power is split by- said splitting mean 3 between said first optical signal power and said second optical signal power, said split between said first optical signal power and said second optical signal power being dependent upon said wavelength of said source optical signal; a first light detecting diode having an anode and cathode for receiving said first optical signal and- for providing a first detector signal indicative of said first optical signal power; a second light detecting diode having an anode and a cathode for receiving said second optical signal and for providing a second detector signal indicative of said second optical signal power; and control means comprising connecting means fcr connecting the anode of said first light detecting diode to said cathode of said second light detecting diode so that the first and second detector signals are subtracted from one another, said control means providing an output signal representing the difference between said first detector signal and said second detector signal which is transmitted to said source means for controlling said wavelength of said source optical signal in response to said first and second detector signals.

2. The apparatus of claim 1 wherein said control means further comprises curreat control means connected to said connecting means and to said source means, said current control means being responsive to said output signal representing said difference between said first detector signal and said second detector signal for --«*