Method and arrangement for actuating a wavelength-tunable laser diode in a spectrometer

Abstract

Method in which, in order to actuate a wavelength-tunable laser diode in a spectrometer, a power-time function is predetermined instead of a current-time function, wherein the laser diode is tuned periodically over a wavelength range in accordance with the power-time function. For this purpose, a current profile (i) with which the laser diode is actuated is determined from the power-time function and measured values of the voltage (u) present at the laser diode.

Claims

1. A method for actuating a wavelength-tunable laser diode in a spectrometer, comprising: predetermining a setpoint variable indicating an electrical power value to be supplied to the wavelength-tunable laser diode as a variable function of time; measuring a voltage present at the laser diode; and driving the laser diode continuously and periodically with a current profile which is determined from (i) the predetermined setpoint variable indicating the electrical value to be supplied to the wavelength-tunable laser diode as the variable function of time and (ii) the measured voltage to periodically tune the laser diode over a wavelength range in accordance with the setpoint variable indicating the electrical value to be supplied to the wavelength-tunable laser diode as the variable function of time.

2. The method as claimed in claim 1, wherein the current profile is generated by a closed-loop control device depending on a system deviation between a power consumption of the laser diode as an actual variable and the setpoint variable of electrical power supplied to the wavelength-tunable laser diode over time; and wherein the voltage present at the laser diode and the current through the laser diode are continuously detected and the power consumption of the laser diode is continuously determined by multiplication of the measured current and voltage values.

3. The method as claimed in claim 1, wherein a computation model is selected which describes a current-voltage characteristic of the laser diode depending on predetermined component parameters of the laser diode; and wherein the voltage present at the laser diode and the current through the laser diode are regularly determined at different times, and component parameters are recalculated based on determined current and voltage values, said component parameters comprising a threshold voltage and a bulk resistance of the laser diode; and wherein the current profile with which the laser diode is driven is determined from the power-time function via the computation model.

4. The method as claimed in claim 3, wherein times at which the voltage present at the laser diode and the current through the laser diode are determined reside outside a time interval of the power-time function; and wherein the laser diode is supplied with different currents at different times and, during this process, a voltage across the laser diode is measured.

5. The method as claimed in claim 4, wherein the different currents are generated as burst signals having different levels.

6. An arrangement for actuating a wavelength-tunable laser diode, comprising: at least one measuring pickup for continuously detecting a voltage present at the laser diode and a current through the laser diode; a multiplier for continuously determining a power consumption of the laser diode by multiplication of a measured current and voltage values; and a closed-loop control device, which generates the current, for driving the laser diode continuously and periodically with a current profile which is determined from (i) the detected voltage present at the laser diode and (ii) a predetermined setpoint variable indicating an electrical value to be supplied to the wavelength-tunable laser diode as a variable function of time.

7. The arrangement as claimed in claim 6, wherein the closed-loop control device includes a controller and a current source controlled by the controller.

8. The arrangement as claimed in claim 6, further comprising: a computation device have a stored computation model describing a current-voltage characteristic of the laser diode depending on predetermined component parameters of the laser diode, the component parameters comprising a threshold voltage and a bulk resistance of the laser diode, and the computation device being configured to recalculate the component parameters based on the measured current and voltage values and to determine a current profile from a predetermined power-time function; and a controllable current source for actuating the laser diode in accordance with the determined current profile; wherein the at least one measuring pickup regularly detects the voltage present at the laser diode and the current through the laser diode at different times.

9. The arrangement as claimed in claim 6, wherein the current profile is generated by the closed-loop control device depending on a system deviation between a power consumption of the laser diode as an actual variable and the setpoint variable of electrical power supplied to the wavelength-tunable laser diode over time; and wherein the voltage present at the laser diode and the current through the laser diode are continuously detected and the power consumption of the laser diode is continuously determined by multiplication of measured current and voltage values.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained below based on exemplary embodiments with reference to the figures in the drawing, in which:

(2) FIG. 1 shows a laser spectrometer with a first exemplary embodiment for an arrangement for actuating a wavelength-tunable laser diode;

(3) FIG. 2 shows the laser spectrometer with a second exemplary embodiment for the arrangement for actuating the laser diode; and

(4) FIG. 3 is a flowchart of the method in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) FIG. 1 shows a laser spectrometer for measuring the concentration of at least one gas component of interest in a sample gas 1, which is contained in a measurement volume 2, such as a measurement cuvette or a process gas line. The spectrometer contains a laser diode 3, whose light 4 falls onto a detector 5 through the sample gas 1. The laser diode 3 is mounted on a temperature-regulated heat sink 6 and is actuated by a current source 7 with a periodically changing current (injection current) i. The intensity and wavelength of the light 4 generated are dependent on the current i and the operating temperature of the laser diode 3. Corresponding to the actuation of the laser diode 3 with the periodically changing current i, a selected absorption line of the gas component of interest is sampled periodically in a wavelength-dependent manner. The concentration of the gas component of interest is determined in an evaluation device 8 from the detected absorption at the point of the absorption line and is output as a measurement result 9.

(6) A function generator 10 predetermines a preferably ramp-shaped or triangular power-time function 11, in accordance with which the laser diode 3 is intended to be tuned periodically over a wavelength range for sampling the absorption line. The current i flowing through the laser diode 3 and the voltage u present at the laser diode 3 are measured continuously using suitable measuring pickups 12, 13 and supplied to a multiplier 14 to continuously determine the present power consumption 15 of the laser diode 3. The power-time function 11 and the measured power consumption 15 are supplied as setpoint and actual variables, respectively, to a closed-loop control device 16, which contains a subtractor 17 for determining the system deviation between the setpoint variable and the actual variable, a controller 18, such as a proportional-integral-derivative (PID) controller, and the current source 7 and, depending on the system deviation, generates the current i for actuating the laser diode 3.

(7) As indicated by the dashed line 19, the controlled variable generated by the controller 18 for the current source 7 can be used instead of the measured current i to determine the present power consumption 15 of the laser diode 3 together with the measured voltage u. The current i therefore does not need to be measured.

(8) FIG. 2 shows a laser spectrometer which is distinguished in terms of the actuation of the laser diode 3 from the exemplary embodiment shown in FIG. 1 in that, instead of the closed-loop control device 16, a computation model stored in a computation device 20 is provided, which computation model models the current-voltage characteristic 21 of the laser diode 3 depending on predetermined component parameters (model parameters) of the laser diode 3, recalculates the component parameters on the basis of present current and voltage values and determines the current profile i for actuating the laser diode 3 from the predetermined power-time function 11.

(9) The current-voltage characteristic 21 can be described in simplified terms as follows:
u=U.sub.S+R.sub.B.Math.i,

(10) where U.sub.S denotes the threshold voltage and R.sub.B denotes the bulk resistance of the laser diode 3. For the power P.sub.L supplied to the laser diode 3, the following then applies:
P.sub.L=(U.sub.S+R.sub.B.Math.i).Math.i

(11) The optical power is negligibly low in comparison with P.sub.L. As a result, P.sub.L also corresponds to the power loss of the laser diode 3.

(12) The two component parameters U.sub.S and R.sub.B can be determined based on two measurements of the current I.sub.1, I.sub.2 and the voltage U.sub.1, U.sub.2:
U.sub.1=U.sub.S+R.sub.B.Math.I.sub.1
U.sub.2=U.sub.S+R.sub.B.Math.I.sub.2

(13) Using the component parameters U.sub.S and R.sub.B thus determined and updated regularly, such as every n-th period, the current i for actuating the laser diode 3 is determined as follows:

(14) $i=\frac{1}{2R_B}.Math.\left(-U_S+\sqrt{U_S^2+4R_B.Math.P_L}\right).$

(15) In order to measure the current I.sub.1, I.sub.2 and the voltage U.sub.1, U.sub.2, the function generator 10 can generate two burst signals 22, 23 of different levels at different times, such as prior to and after every n-th power-time function 11.

(16) FIG. 3 is a flowchart of a method for actuating a wavelength-tunable laser diode (3) in a spectrometer. The method comprises predetermining a power-time function (11), as indicated in step 310. Next, the driving the laser diode (3) is driven with a current profile (i) determined from the predetermined power-time function (11) and measured values obtained from the laser diode (3) to periodically tune the laser diode (3) over a wavelength range in accordance with the predetermined power-time function (11), as indicated in step 320. In accordance with the method of the invention, the measured values are obtained from a voltage (u) present at the laser diode (3).

(17) While there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.