Continuously tunable laser oscillator

Abstract: A laser oscillator comprises a control unit (1) and a laser unit (9), in which the control unit (1) generates control signals (I, T) for setting the power and the wavelength of the laser light generated by the laser unit (9) on the basis of wavelength values fed with the aid of a keyboard and the power of the laser light. In prior-art laser oscillators the relation between power and wavelength, on the one hand, and the two control quantities, on the other hand, is stored in a Table in the form of a ROM or RAM. For avoiding a very large Table required for a large number of possible values of the wavelength or the power, no Table is used in the laser oscillator according to the invention, but the control signals are calculated with the aid of an expression stored in the microprocessor 4.

Description: The invention relates to a laser oscillator, comprising a laser unit and a control unit for applying at least one control signal to at least one control input of the laser unit on the basis of a desired value of at least one physical quantity of the laser light. A laser oscillator of this type is known from the conference record "One THz Digital random access high resolution optical frequency synthesizer providing cold-start operation from a frequency reference" by B. Glance and O. Scaramucci at GLOBECOM '90 IEEE Global Telecommunications Conference & Exhibition, San Diego, California, December 2-5-1990, pp.766-767. Such laser oscillators are used, for example, in transmitters or receivers for coherent optical transmission systems, in spectroscopy equipment or in test equipment for optoelectric converters. For transporting a baseband signal via a glass fibre in coherent optical transmission systems, the light signal coming from a transmitting laser can be amplitude, frequency or phase modulated by the baseband signal before it is fed to the glass fibre. For demodulating the light signals at the receiver with the aid of current electronic components, it is necessary to convert the light signal which has a very high frequency (for example, 10¹⁴ Hz) to a much lower intermediate frequency of, for example 10⁹ Hz. For this purpose, the received light signal is combined in the receiver with a local laser-generated light signal with the aid of an optical coupling element. This combination provides an optical signal having amplitude variations due to interference between the two input signals of the coupling element. These amplitude variations have a frequency which is equal to the difference frequency between the frequency of the received light signal and that of the locally generated light signal. A photodiode is then used for converting the optical amplitude variations into an electrically processable intermediate-frequency signal. For simultaneously transporting more than a single signal via a glass fibre, lasers which are tunable over a large frequency range (for example, 500 GHz) are used in both the transmitter and the receiver. As a result, more transmitters and receivers may communicate via the same glass fibre without causing interference to one another. In prior-art laser oscillator the physical quantity of the laser light is the frequency of this light. Another possible physical quantity is the power of the laser light. In prior-art laser oscillator the laser unit has two control inputs, a first control input determining the current flowing through the laser diode and a second control input determining the temperature of the laser diode. The control unit comprises a Table in the form of a ROM or a RAM in which the values of the control signals are stored as functions of the desired value of the physical quantity (in this case the frequency of the laser light). As a result, the physical quantity of the laser light may be modified in steps. With a predetermined range of the physical quantity of the laser light, the smallest possible step size with which this physical quantity can be modified is determined by the size of the Table. A disadvantage of prior-art laser oscillator is that when the physical quantity of the laser light has a large range and when also a small step size is desired, the Table is to be rather large, so that also a RAM or ROM is required to have a large memory capacity. It is an object of the invention to provide a laser oscillator of the type mentioned in the opening paragraph, in which a large range of the physical quantity with a small step size is possible without the need for a RAM or ROM with a large memory capacity. For this purpose, the invention as claimed in Claim 1 is characterized in that the control unit comprises calculation means for calculating according to a mathematical expression stored in the control means, the control signal from the desired value of the physical quantity of the laser light while there exists an associated control signal value for each value of the physical quantity of the laser light within the working range of the laser unit. Because the laser oscillator comprises calculation means for calculating the control signal on the basis of the desired value of the physical quantity, no Table is needed any more for looking up the value of the control signal with reference to the desired value of the physical quantity. However, it is also conceivable for the calculation means to perform an interpolation between setting points stored in a (small) Table. Moreover, an additional advantage is that the physical quantity may now be adjusted to any desired value within the range of the physical quantity. The relation between the control signal and the wavelength or the power of the laser light respectively, will generally be such that no explicit expression may be found for the value of the control signals as a function of the power and the wavelength. The value of the control signals will then have to be found by implementing numerical methods. Such numerical methods are described, for example, in the book entitled "Einführung in die Numerische Mathematik I" by Josef Stoer, Springer Verlag, ISBN 0-387-057501, Chapter 5. An embodiment of the invention is chatacterized in that according to said mathematical expression the physical quantity is defined by a sum of terms, at least one of said terms comprising a power of the control signal. By approximating the relation between the control signal and the physical quantity by way of a truncated power series, an explicit expression may be found for the control signal as a function of the physical quantity, which expression considerably reduces the necessary amount of calculations for the determination of the value of the control signal. An embodiment of the invention is characterized in that the laser oscillator comprises measuring means for supplying to the control unit a measurement signal which is a measure for the value of the physical quantity of the laser light, the control unit comprising adaption means for adapting the mathematical expression stored in the control means so as to reduce the difference between the desired and measured values of the physical quantity By comparing the value of the physical quantity of the laser light to a reference value and adapting the relation between physical quantity and control signal in response to the difference signal, it is possible to strongly increase the absolute accuracy of the physical quantity setting. A further embodiment of the invention is characterized in that the number of control inputs of the laser unit exceeds by a predetermined number the number of physical quantities of the laser light, the control signals satisfying said predetermined number additional auxiliary conditions, for preventing an undesired change of oscillation mode by the laser unit. In semiconductor lasers having more than one control input, the control signals on these inputs are often desired to have a certain relation. An example of this is, for example, a so-called Distributed Bragg Reflector (DBR) laser. Such a laser comprises an amplifier section L, a transmission line section P and a (reflecting) Bragg section B. A current I a is applied to the amplifier section L, which current is to exceed a certain threshold to make optical amplification possible. Control signals I p and I B are applied to the transmission line section P and the Bragg section B respectively. The control signal I p in the transmission line section determines the breaking index thereof and thus the phase rotation of the transmission line section. The control signal I B in the Bragg section influences the breaking index thereof and thus determines the phase of the light reflected by the Bragg section. For laser oscillation at a desired frequency the sum of the phase rotations in the Bragg section and the transmission line section is to be equal to K·2·π (K ε ), while the marginal condition that the phase rotation in the Bragg section B is to be nearest to π/2 is to be satisfied. In the event of large deviations from this marginal condition, undesired frequency jumps as a result of a sudden change of the oscillation mode of the laser during the laser tuning may occur. The change of the oscillation mode takes place because K changes in a value for which the phase rotation in the Bragg section B is nearer to π/2. By forcing suitably selected auxiliary conditions onto the control signals I b , and I p , the two conditions may be satisfied, so that the undesired frequency jumps do not occur. If the laser unit has m control inputs and if n marginal conditions are forced onto the control signals, m-n physical quantities of the laser light may be determined. The entire system is then described by m equations (n marginal conditions and m-n expressions for the physical quantities) with m unknowns (the control signals). Also in this case this system of equations will generally have to be solved with the aid of numerical methods. The invention will now be further explained with reference to the drawing Figures, in which: Figure 1 shows a block diagram of a laser oscillator according to the invention; Figure 2 shows a flowchart of a program for the processor 4 in Figure 1 to control the laser unit 9; Figure 3 shows a flowchart of a program for the processor 4 in Figure 1 for the calibration of the laser oscillator; and Figure 4 shows a relationship between two control signals of the laser unit 9 which has to be satisfied to avoid frequency jumps. In Figure 1 a first control output of the control unit 1, carrying control signal I, is connected to a first control input of the laser unit 9. A second control output of the control unit 1, carrying output signal T, is connected to a second control input of the laser unit 9. The control unit 1 comprises calculation means in this case constituted by a microprocessor 4. A keyboard 2 is connected to this microprocessor 4. A first output of the microprocessor 4 is connected to a digital-to-analog converter 6. The output of the digital-to-analog converter 6 constitutes the output of the control unit and carries output signal I. A second output of the microprocessor 4 is connected to a digital-to-analog converter 8. The output of the digital to analog converter 8 constitutes the output of the control unit and carries output signal T. The first input of the laser unit 9 is connected to a first input of a subtracter 10. The output of the subtracter 10 is connected to the input of an amplifier 14. The output of the amplifier 14 is connected to a first connection of a measuring resistor 21 and to a first input of a differential amplifier 27. A second connection of the measuring resistor 21 is connected to the anode of a laser diode 20 and to a second connection of the differential amplifier 27. The cathode of the laser diode 20 is connected to a point of constant potential further to be referenced earth. The output of the differential amplifier 27 is connected to a second input of the subtracter 10. The second input of the laser unit 9 is connected to a first input of a subtracter 12. The output of the subtracter 12 is connected to the input of an amplifier 16. The output of the amplifier 16 is connected to the input of a Peltier element 22. The Peltier element 22, together with the laser diode and an NTC resistor 24 is accommodated in a housing 18 in which these three elements are thermally coupled. The NTC resistor 24 is included in a temperature measuring circuit (for example a bridge), whose output is connected to the second input of the subtracter 12. The light generated by the laser is available at the output 23. For calibrating the laser oscillator, the output 23 of the laser oscillator may be coupled to the input of a wavelength meter 26. The (digital) output of the wavelength meter 26 is then connected to a further input of the microprocessor 4. In the laser oscillator as shown in Figure 1 the physical quantities to be set are the wavelength and the power of the light to be emitted by the laser oscillator. The desired values of these quantities may be loaded into the microprocessor 4 by means of the keyboard 2. The microprocessor 4 calculates the values of the control signals I and T on the basis of the desired values of the power and the wavelength. The control signal I determines the current flowing through the laser diode 20, while the control signal T determines the temperature of the laser diode. The feedback amplifier 14 has a large gain factor so that the amplifier will adjust its output signal in such a way that its input signal, and thus the output signal of the subtracter 10, will be very small. The output signal of the differential amplifier 27 will then be equal to the signal I. As a result, the voltage across the resistor 21 is directly proportional to the signal I, so that the current flowing through the laser diode will be directly proportional to the quotient of the signal I and the resistance of the resistor 21. The feedback amplifier 16 likewise has a large gain factor, as a result of which this amplifier will adjust its output signal in such a way that its input signal and thus the output signal of the subtracter 12 will be very small. Consequently, the output signal of the temperature measuring circuit is equal to the signal T, so that the temperature of the laser diode will be proportional to the control signal T. The following can be written in approximation for the relation between the control signals I and T and the wavelength and power respectively, of the laser light: Herein λ R is the wavelength of the laser light if the control signal I is equal to a reference value I R and if also the control signal T is equal to a reference value T R . P R is the power of the laser light if the control signal I is equal to the reference value I R and if also the control signal T is equal to the reference value T R . Based on the desired values of λ and P, there can be derived for the control signals I and T: From (2) there can be observed that two solutions are possible for I and T. For the setting of the laser unit the value of I is taken now, while both the signal I and the associated signal T are situated in a permitted interval. With the aid of the wavelength meter 26 it is possible to calibrate the relation between the control signals and the wavelength or the power of the laser light respectively. This may be effected regularly for which the wavelength meter is constantly switched over between a number of laser oscillators, so that more laser oscillators can be calibrated with a single wavelength meter. Such a system is known, for example, from United States Patent 4,942,568. Alternatively, it is possible for the calibration to take place only incidentally, for example, once a year. If the relation between the control signals and the wavelength of the laser light and the relation between the control signals and the power of the laser light is expressed by (1), the values of the constants A, B, C, D and E are to be adapted during the calibration. For this purpose, the wavelength of the laser light belonging to three different combinations of values of the control signals is measured. For determining the constants D and E, it is only necessary to measure the power of no more than two different combinations of control signals. The new values of A, B, C, D and E are then determined in the following manner. A first measurement of the wavelength is performed with a value of the control signal I equal to I R and with a value T a of the control signal T. If the measured wavelength is then equal to λ₁ and the measured power is equal to P₁, the following holds for the constants A and D: A = λ₁-λ R T a - T R , D = P ₁- P R T a - T R Subsequently, a second and a third wavelength measurement is performed for which the control signal T is equal to T R and I adopts the values I R + δ and I R - δ respectively. For the measured wavelengths λ₂ and λ₃ respectively, and the measured power P₂ the following holds: From (4) the following can be simply derived for the values of B and C: In Figure 2 the numbered instructions have the connotations as stated in the Table below. In instruction 30 all variables required are initialized. In instruction 32 the desired values of the wavelength λ and the power P of the laser light are read from the keyboard 2. In instruction 34 the two values I₁ and I₂ and the associated values T₁ and T₂ are calculated. This calculation is made on the basis of (2). In instruction 36 it is checked whether the value I₁ is situated in the interval [I min , I max ] and whether the value T₁ is situated in the interval [T min , T max ]. This check is necessary because the values I₁ and I₂ are solutions to a quadratic equation and because it cannot be determined in advance which of the two values leads to a solution according to which both the value of I and the associated temperature for the laser unit is permissible. If both I₁ and T₁ are situated in the permitted interval, the control signals I and T respectively, are made equal to I₁ and T₁ respectively, in instruction 40. If either of the values I₁ or T₁ is not situated in the permitted interval, it is checked in instruction 38 whether the value I₂ is situated in the interval [I min ,I max ] and whether the value T₂ is situated in the interval [T min ,T max ]. If this condition is satisfied, the control signals I and T respectively, are made equal to I₂ and T₂ respectively, in instruction 42. If either of the values I₂ or T₂ is not situated in the permitted interval, an error message is given. In Figure 3 the numbered instructions have the connotations to be described in the Table below. In instruction 50 the values of I and T are set at I R and T a respectively. Then there is a certain waiting period during which the laser unit is given the opportunity to adopt the temperature defined by the control signal T. In instruction 54 the wavelength and the power are measured and assigned to the variables λ₁ and P₁ respectively. In instruction 56 the values of A and D are calculated according to (3). In instruction 58 the values of I and T are set at I R +δ and T R respectively. In instruction 60 there is another waiting period before in instruction 62 the wavelength and the power of the laser light are measured and the values are stored under variables λ₂ and P₂ respectively. In instruction 64 the control signal I is made equal to the value I R -δ. Subsequently, in instruction 66, the wavelength is measured and assigned to the variable λ₃. In instruction 68 the constants B, C and E are calculated according to (5). Figure 4 shows a characteristic of a DBR laser, the boundaries of the different oscillation modes being plotted against the tuning signals on the two frequency control inputs. These boundaries are denoted by the character B. In the hatched area the oscillation mode is not determined unambiguously, but is equal to the oscillation mode when the boundary B is exceeded. In Figure 4 two tuning curves 1 and 2 are shown. They markedly show that when tuning is effected according to tuning curve 1 this tuning curve exceeds the boundary between different oscillation modes several times (point X and point Y), so that an undesired frequency jump will occur. By selecting the relation between the two control signals in accordance with curve 2, it is possible to avoid exceeding the boundary between different oscillation modes. The relation between the two control signals is then a marginal condition the control signals are to satisfy.

Claims: Laser oscillator, comprising a laser unit (9) and a control unit (1) for applying at least one control signal (I,T) to at least one control input of the laser unit (9) on the basis of a desired value of at least one physical quantity (P,F) of the laser light, characterized in that the control unit (1) comprises calculation means (4) for calculating according to a mathematical expression stored in the control means, the control signal from the desired value of the physical quantity of the laser light while there exists an associated control signal value for each value of the physical quantity of the laser light within the working range of the laser unit. Laser oscillator as claimed in Claim 1, characterized in that according to said mathematical expression the physical quantity (P,F) is defined by a sum of terms, at least one of said terms comprising a power of the control signal(I,T). Laser oscillator as claimed in Claim 1 or 2, characterized in that the laser oscillator comprises measuring means (26) for supplying to the control unit (1) a measurement signal which is a measure for the value of the physical quantity of the laser light, the control unit (1) comprising adaption means for adapting the mathematical expression stored in the control means so as to reduce the difference between the desired and measured values of the physical quantity (P,F). Laser oscillator as claimed in Claim 1, 2 or 3, characterized in that the number of control inputs of the laser unit (9) exceeds by a predetermined number the number of physical quantities of the laser light, the control signals satisfying additional auxiliary conditions, for preventing an undesired change of oscillation mode by the laser unit, whereas the number of auxiliary conditions correspond to said predetermined number.

Inventors: WONG CHING KWOK, WONG, CHING KWOK
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01S3
IPC codes: H01S5