Research on Spectroscopy Modulation of a Distributed Feedback Laser Diode Based on the TDLAS Technique

Laser current and temperature control circuits have been developed for a distributed feedback laser diode, which is applied as the light source of a tuneable diode laser absorption spectroscopy system.,e laser’s temperature fluctuation can be limited within the range of −0.02 to 0.02°C, and good operation stability was observed through 15 hours of monitoring on the emitting wavelength of the laser. Response time of temperature modulation was tested which is suitable for the tuning requirements of gas detection systems. Laser current can be injected within the range from 40 to 80mA. In addition, a linear power supply circuit has been developed to provide stable and low-noise power supply for the system. ,e physical principles of laser modulation theory are discussed before experiments. Experiments show that the output wavelength of the laser can be tuned accurately through changing the working current and temperature.,e wavelength can be linearly controlled by temperature at 0.115 nm/°C (I� 70mA) and be controlled by current at 0.0140 nm/mA (T� 25°C). ,is is essential for the tuneable diode laser absorption spectroscopy systems. ,e proposed cost-effective circuits can replace commercial instruments to drive the laser to meet the requirements of methane detection experiments. It can also be applied to detect other gases by changing the light source lasers and parameters of the circuits.

Tuneable diode laser absorption spectroscopy (TDLAS) technology is widely used in the field of trace gas detection, among many of the optical detection techniques [17][18][19][20]. In TDLAS systems, near-infrared (NIR) and mid-infrared (MIR) lasers are generally applied to be light sources. Distributed feedback (DFB) lasers are suitable for utilization in TDLAS systems and are especially used in the field of methane detection for the narrow line width and highly selective and reliable features [21][22][23]. In addition, the cost of DFB lasers is relatively much lower than that of midinfrared lasers and DFB lasers have the potential of remote multipoint probing by using optical fibre.
Tuneable laser diodes have three major parts: the excited region of active gain region and resonant cavity, wavelength tuning unit, and wavelength locking unit. First, wavelength can be changed by the resonant cavity. en, frequency and phase of output wavelength can be modified and stabilized. e wavelength tuning unit is responsible for changing and selecting output wavelength. And the wavelength lock unit can stable the selected output wavelength [24]. Lasers can be catalogued according to their structures and processing technology. DFB lasers are tuneable laser diodes and are adopted in this paper. e output wavelength of DFB lasers is determined by the grating period Λ and effective refractive index. Due to the reason that the effective refractive index is influenced by temperature, the output wavelength can be determined by temperature. In this way, the maximum tuning scope of a DFB laser is determined by the maximum scope of grating material's effective refractive index [25]. Generally, the current-wavelength tuning ratio is about 0.01 nm/mA. e temperature-wavelength tuning ratio is much bigger and is about 0.1 nm/°C. DFB lasers are controlled by using laser driver devices that can set the working current and temperature. In this way, experiments based on TDLAS are commonly utilizing commercial instruments including laser current drivers and temperature controllers to satisfy the experimental requirements. Besides, a signal generator is also a type of essential instrument for a laser's output signal control. By utilizing the commercial instruments mentioned above, lasers can be controlled effectively to meet the requirements of gas-sensing experiments based on infrared absorption spectroscopy techniques [26,27]. However, there are still some drawbacks of using the commercial instruments. For instance, these systems are generally expensive and large. ese drawbacks are practical barriers for industrialization of TDLAS-based gas-sensing technique, and these problems should be solved.
In this paper, a cost-effective DFB laser driver was developed and experiments were demonstrated.
is laser driver is made of self-developed circuits. It has the capability of controlling laser temperature, driving current, and modulating essential signals based on the TDLAS technique. e modulated DFB laser can be applied as the radiation source of gas detection system. In order to provide stable and low-noise power supply, a linear power supply circuit has been developed to satisfy the system requirements. Spectroscopy measurement has been carried out to evaluate and optimize the performance of the DFB driving circuits and control algorithm. e applied gas-sensing theory is briefly discussed only to guide the audience and define units. en, details of the key modules developed in this paper are demonstrated with individual tests. In addition, the modulation theory of lasers by current and temperature has been discussed in order to provide theoretical basis for circuits' design and experiments. Finally, the spectroscopy is measured by using the self-developed laser driver and the results are discussed in the end of this paper.

Theory of Spectroscopy Modulation
Modulation and implementation of a near-infrared DFB diode laser are generally easier and cheaper than those of a 3.31 μm DFB diode laser, though CH 4 molecules have larger absorption intensity at the wavelength of 3.31 μm, which is in the middle infrared domain. e laser diode adopted in this paper is a near-infrared distributed feedback laser diode, and its centre wavelength is shown in Figure 1. DFB lasers are mainly manufactured in the near-infrared domain and middle-infrared domain, and the output signal can be transmitted by using optical fibres. In this paper, the DFB laser's absorption band around 1.65 μm is selected in CH 4 detection. Figure 1(a) depicts the absorption spectrum of CH 4 molecules around 1.654 μm, and the intensity is at the level of 10 −21 cm −1 /(molecule cm −2 ). As can be seen from the comparison among Figures 1(a)-1(c), since the absorption intensities of H 2 O and CO 2 are both 4 orders smaller than that of CH 4 , H 2 O and CO 2 cause extremely slight effect on CH 4 detection, and the detailed absorption band used in detection is shown in Figure 1(d) for a clearer illustration. erefore, the centre wavelength of the DFB laser can be tuned across the absorption line of CH 4 by changing its working current and temperature and the optical power loss caused by H 2 O, CO 2 , and other gases can be ignored. Hence, detection selectivity of the CH 4 TDLAS system can be guaranteed.
e schematic of this TDLAS-based methane detection system is shown in Figure 2. e proposed system is a typical TDLAS-based system, which consists of electrical circuit boards, DFB laser, fibre optic beam splitter (FOBS), optical attenuator (OA), InGaAs detectors, and input/output interfaces. e DFB laser, which is 14-pin butterfly-packaged, has a central wavelength around 1.654 μm. A digital signal processor (DSP) chip (TMS320F28335) is applied as the main controller of the developed DFB driver. It controls the laser's temperature, driving current, and modulating signals by sending signals to corresponding circuits such as TEC controlling circuit and DAC circuit. Meanwhile, it also processes signals that are related to gas concentration. Two InGaAs detectors are applied to convert optical signal to electrical signal. e yellow lines in Figure 2 represent optical fibres. e laser driving functions including temperature control, current control, and signal modulation are included together in this figure and are proposed in this paper. e fundamental of absorption spectroscopy is based on the attenuation of spectral intensity (W/m 2 ) occurring when radiation interacts with species molecules and is governed by the Beer-Lambert law, which relates the transmitted intensity I(υ ) to the incident intensity I 0 (υ) as follows: where α(υ, t) (cm −1 ) is the absorption coefficient at frequency υ (cm −1 ), C is the mole fraction of the absorbing specie, and L is the optical path length. In application of TDLAS with WMS, the DFB laser is tuned with a lowfrequency repetitive ramp waveform with the period of T ramp . is has the effect of repetitively ramping the laser intensity and the laser wavelength across the absorption feature. In addition, a high-frequency sine wave signal with an angular frequency of ω sin is superimposed on the repetitive injection current ramp to generate an additional high-frequency modulation [20]. e expressions of the ramp signal and the sine signal are represented in equations (2) and (3), respectively.

Laser Temperature Controller.
Generally, there are four ways to change the frequency of tuneable lasers as shown in Table 1. First, the plasma effect of carrier is caused by injection current. Second, electric field effect is known as the quantum confined Stark effect (QCSE). ird, temperature effect is widely adopted in TDLAS systems. Finally, mechanical effect is commonly adopted for the vertical cavity surface-emitting laser (VCSEL) [28][29][30][31]. It can be seen in Table 1 that the temperature effect is based on changing the effective refractive index in order to change the frequency. It is effective to modulate the DFB laser, and it is fordable to develop through the thermo electric cooler (TEC) control circuits for temperature controlling. e resonance condition of the laser diode can be expressed as shown in Here, λ is the laser wavelength, m is the resonant modes factor, n eff is the effective refractive index of active medium, and L is the effective length of the active dielectric resonator. erefore, the relative variation of wavelength modulation can be shown as Here, wavelength modulation can be achieved by using current modulation or temperature modulation in order to change the parameters including Δn eff , ΔL, and Δm. Current modulation is based on changing the injection current on electrodes in order to change the density of free carriers. And wavelength modulation can be performed by the variation of effective refractive index caused by plasma effect. Temperature modulation is similar to current modulation, and the effective refractive index can be changed by temperature variation in order to change the wavelength. In this paper,   International Journal of Optics both methods are applied to modulate wavelength by using the developed circuits.
Temperature control for the DFB laser is crucial for gas detection due to the fact that the laser's central wavelength and optical power are determined by the laser's operation temperature and working current.
us, a temperature controller circuit was developed to meet the requirements of control features including stable, accurate, and quick response. Based on the Peltier effect, the laser's temperature can be set through the TEC module, which is integrated in the DFB laser. e DFB laser's temperature can be obtained by an inside-packaged thermistor with negative temperature coefficient (NTC). A high-efficiency and reliable monolithic TEC controller, ADN8831, is adopted to drive the TEC. It sets the temperature of the laser diode in a programmable way and maintains the temperature stable by using an analog PID control loop. e developed circuit is shown in Figure 3. It is a four-layer PCB, and the internal layers are prepared for power supply and GND. e parts on the board were soldered manually. e temperature compensation network is shown in Figure 3. e values of R LOW , R MID , and R HIGH can be determined, and the voltage V SET , which determines the laser temperature, can be calculated using where R LOW , R MID , and R HIGH depend on the temperature range, R TH is the resistance of the thermistor, V REF is the reference voltage, R 1 and R 2 are calculated using equations (8) and (9), and R 3 represents the compensation resistor. e schematic of temperature setting and compensating network is shown in Figure 3, where the resistors and capacitors, CD, RD, RI, CI, RP, and CF, set the analog PID loop. In order to evaluate the performance of temperature control, an evaluation board was developed as shown in Figure 3. Experiments were carried out to investigate the temperature control performances. Figure 4 shows the stability of the temperature control module. During a period of 15 hours, the operation temperature of the DFB laser is set as 24.55°C. e practical temperature is sampled by an analogto-digital circuit, and a random period of 100 minutes is selected to see the details. It can be seen that the temperature is fluctuated within the range of ±0.02°C, which is small enough to avoid laser wavelength fluctuation.
A response time experiment was carried out to evaluate the performance of the laser temperature control circuit, and the result is shown in Figure 5. e response time test was performed by increasing and decreasing the temperature with the step of 1°C. It can be seen in the figure that the response time is less than 8 seconds and the overheating and overcooling range is within 3°C without repeating thermal vibration. According to Table 1, laser frequency can be controlled by temperature and the response time is at the millisecond level. However, it requires more time to stabilize the working temperature of the laser after changing its value. It can be acceptable to have the temperature stabilized in the period of a few seconds. erefore, experiment results suggest good performance of the developed laser temperature controller.

Linear Power Supply.
In order to guarantee the operation stability and protect the DFB laser, a stable and lowripple linear power supply was developed and integrated, whose photo is shown in Figure 6. is power supply module possesses features including voltage regulation and low ripple, and these features are indispensable, especially for the temperature control module. e measured current variation is less than ±2 mV. Electrostatic discharge (ESD) protection and surge protection are also put into use in this power supply system to protect the DFB laser. ere are two circuit boards in this part. e parts on the boards were soldered manually. e output voltage has been set as 5 V and ±12 V as shown in the photo.

Laser Current Controller.
e DFB laser applied in this detection system is provided by the Institute of Semiconductors, Chinese Academy of Sciences (ISCAS). Its centre emitting wavelength is around 1654 nm, and the laser photo is shown in Figure 7. It is 14-pin butterfly-packaged and has been designed for high-sensitivity optical sensor development. A negative temperature coefficient thermistor and a thermoelectric cooler are packaged inside for temperature monitoring and controlling. e threshold current is 17.23 mA, and the optical output power is 5.34 mW, while the injection current is 60 mA at 25°C. e driving current is set within the range of 40-80 mA, and the current varying step is 0.02 mA. International Journal of Optics e integrated board has been developed contenting functions including laser current control, laser temperature control, laser signal modulation, and communication functions as shown in Figure 7. e main controller of this circuit is a DSP chip, which is responsible for processing the calculations of laser current control, laser temperature control, and other functions. is 4-layer circuit board was also selfdeveloped by using the software Cadence. e output power of the laser is detected by using an optical power meter as shown in Figure 7. It can be seen that the optical power is 10.4 mW, which is the maximum optical power of this DFB laser diode. Further experiments were carried out to evaluate the developed circuits by using this board.

Laser Temperature Robust Test.
In order to evaluate the laser temperature control performance of the developed circuit, a laser temperature robust test has been carried out as shown in Figure 8. In this test, the laser is constantly heated by using a heat gun to simulate its working condition.
is test repeated twice from the beginning to the end in a period of 200 seconds, and the laser temperature control circuit was switched. On the one hand, as shown in Figure 8(a), the black line demonstrates the laser temperature without the control circuit. It can be seen that the laser temperature maintains at about 27°C, which is the room temperature, and it increases sharply continually when it was heated at the 80th second. On the other hand, the test was repeated again with the temperature control circuit as shown of the red line in Figure 8(a). It can be seen that the laser temperature maintains at the same level. e enlarged curve from 80 to 200 second is shown in Figure 8(b). It can be seen that the laser temperature started to fluctuate when it was heated, but the temperature has been controlled to its target by the analog PID circuit. In this way, the output centre wavelength of the laser can be maintained with the laser temperature control circuit.

Spectroscopy Test with Varying Laser Temperature.
e centre wavelength of the laser is determined by the working current and temperature according to the previous descriptions. e relation between the centre wavelength and the temperature has been tested by using the developed circuit, and the results are shown in Figure 9. e laser current was set by the DSP circuit via DAC and VCCS circuits. In this figure, it can be seen that the laser wavelength increases linearly corresponding to the increasing  Temperature set circuit International Journal of Optics laser temperature, where the current is set to be 70 mA. It can also be seen that the optical power of each curve is slightly varied as the centre wavelength is shifted linearly. e factor between wavelength and temperature is about 0.115 nm/°C. is reveals that laser temperature determines the centre wavelength significantly and determines the optical power slightly. e result as shown in the figure proves good stability and linearity of the temperature controller.

Spectroscopy Test with Varying Laser Current.
en, the driving current of the laser was changed and the corresponding spectrum was measured as shown in Figure 10; current modulation causes wavelength shifting while the temperature is maintained at 25°C. e current is increased from 40 mA to 80 mA with the step of 10 mA. Meanwhile, the optical intensity is also largely affected by tuning the driving current. While the driving current increases, the optical intensity rises proportionally. e factor between In gas-sensing experiments, the wavelength of the laser is tuned close to the absorption line by adjusting laser temperature to a specific value firstly. en, the laser temperature is fixed at this point during the experiment. e driving current is periodically changed in order to tune the wavelength. In this way, the wavelength periodically scans across the gas absorption line for the absorption phenomenon. e current is modulated 2000 times per second, which is performed by a DSP timer. e test result proves good efficiency and performance of the developed laser current control circuit.

Spectroscopy Stability Test.
Experiments of spectroscopy stability were carried out to investigate the current driving performance along with the temperature control module. Spectroscopy stability is crucial for TDLAS experiments due to the fact that the shifted centre wavelength of the laser can result inaccuracy gas absorption measurement. e curves of the laser output wavelength versus the injection current is shown in Figure 11. It can be seen that the wavelength under constant temperature and injection current is repeatable. e wavelength was measured five times every 10 minutes, and the curves indicate the good stability and replication of the DFB driver. International Journal of Optics

Conclusions
Based on the TDLAS technique, laser current control and temperature control circuits have been developed to control the output wavelength of the laser diode. e wavelength of the laser diode is at 1654 nm, which is applied for methane detection. Laser current and temperature control circuits have been developed and tested accordingly. In addition, a linear power supply circuit has been developed to ensure the stability of the system. Spectroscopy measurements have been performed by changing the laser's current and temperature. Experiments show that the centre wavelength of the laser can be effectively tuned by using the self-developed circuit instead of the commercial instruments. e laser's output wavelength can be controlled by temperature at 0.115 nm/°C and be controlled by current at 0.0140 nm/mA. e laser's temperature fluctuation can be limited within the range of −0.02 to 0.02°C, and good operation stability was observed through 15 hours monitoring on the emitting wavelength of the laser. e laser current can be injected within the range from 40 to 80 mA, and the centre wavelength of the laser was measured accordingly. e proposed circuit can also be applied to drive other DFB lasers for gas detection experiments by changing the parameters and components of the circuit.

Data Availability
Related data are all included in this article as figures.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.  International Journal of Optics