Characterization of a 976 nm High Power Diode Laser Module

Paper: ASAT-13-TE-23 th 13 International Conference on AEROSPACE SCIENCES & AVIATION TECHNOLOGY, ASAT- 13, May 26 – 28, 2009, E-Mail: [email protected]
Author: Aubrey Cain
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Paper: ASAT-13-TE-23 th

13 International Conference on AEROSPACE SCIENCES & AVIATION TECHNOLOGY, ASAT- 13, May 26 – 28, 2009, E-Mail: [email protected] Military Technical College, Kobry Elkobbah, Cairo, Egypt Tel : +(202) 24025292 – 24036138, Fax: +(202) 22621908

Characterization of a 976 nm High Power Diode Laser Module A.F. EL-Sherif*, M.F. Hassan**, Y. H. El-Sharkawy***, M. Kh. El-Tahlawy† Abstract: A complete characterization for the diode laser module operating at 976 nm is required for different applications; such as pumping the doped fiber in the laser, industrial and medical applications. In this paper a complete characterization for a high power diode laser module with fiber pigtail is presented. We obtained an optical power up to 6 Watt (cw). The electrical characterization of the diode laser module was examined by measuring the dependence of the laser driving current on the operating voltage. The optical characterization was investigated at different temperatures. These included the laser output power versus the driving current, the output optical power versus electrical input power, the laser output spectrum, the laser beam profile and the dependence of the output power on the temperature. The measured diode laser parameters at 25oC were (0.3A) threshold current, (35.84%) overall slope efficiency and (977.34 nm) central wavelength with line width of (1.3 nm) at FWHM. The variation of the output power, spectrum with temperature are presented.

1. Introduction: The developing of TEM00 laser mode with high efficiency, high output power, good spatial beam profile and good stability is highly desired. This will make it suitable for using in material processing and other scientific applications. One of these applications is pumping other laser crystals (Diode-Pumped-Solid-State Laser "DPSSL") and (Diode-Pumped-FiberLaser "DPFL"). The new technique such as High Power Diode Laser modules for pumping solid state lasers and fiber lasers is a reliable technique in developing TEM00 laser output [13]. High power laser diode arrays have found wide applications in manufacturing, defense, communications, and pumping of solid state lasers and fiber lasers. Diode laser arrays for high continuous wave (CW) power operation are typically sold as one dimension array of emitters, known as a laser bar. The array contains a number of emitters, which are spaced physically apart to ensure sufficient thermal isolation between adjacent elements.

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Egyptian Armed Forces, [email protected] Egyptian Armed Forces, [email protected] *** Egyptian Armed Forces, [email protected] † Egyptian Armed Forces, [email protected] 1/10 **

Paper: ASAT-13-TE-23

Since the light emission from a diode laser is highly divergent, lenses are used to collimate the light. In the vertical (fast) direction, the light is collimated by a single cylindrical or aspheric rod. In the lateral (slow) axis, a specially designed optic collimates the output from each emitter individually. The diode laser bar is mounted onto a heat sink to remove the waste heat generated by the device under operation. The cooling for the high power diode laser bars can be passive or active. A typically passively cooled diode is sold on a CS mount, a standard package that is compatible with a thermo-electric cooler (TEC) based mounting fixture. Laser diode characterization include five categories : •Electrical : Measurement of light output, voltage drop, and operating current. •Spatial : Output light intensity profile in the far and near the field and pointing angle of radiation pattern. •Spectral : Spectral data acquired to calculate spectral width, center wavelength and to observe mode structure. •Optical : Measurement of astigmatism and other wave front errors. •Dynamic : Measurement of noise, intermodulation, distortion, rise time, fall time, chirping and so on. •Thermal : The variable of the above characteristics with temperature.

2. Electrical Characterization of the Fiber Coupled Diode Module: a. Experimental setup: The experimental setup shown in Fig. 1 consists of; laser diode driver with a temperature controller (fully controlled by computer via wireless commands), power supply (output 24V, 10.5A), high power diode laser module (fiber coupled), optical power meter, spectrometer with integrated sphere, thermal camera and laser optical detectors.

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Fig.1 The experimental setup 2/10

Paper: ASAT-13-TE-23

A laser diode driver (UM DiTec 60/8000 TEC OEM) with an integrated highly precise temperature controller (TEC) used for driving diode lasers in cw and pulsed mode as it has an On-Board Oscillator which can be freely configure for pulsation the laser. The driver is completely controllable by RS232 serial interface; where the current, voltage and temperature can freely set, as it will be used to measure the electrical and optical characterization of the high power Diode laser module. Our high power Diode laser module is a fiber coupled @ 976 ± 2 nm (model VDM00231); this Diode has a maximum cw output power up to 5.2 Watt under maximum input driving current at 5.92 Amp. The highest brightness is achieved by transforming the asymmetric radiation from the laser diode into a symmetrical beam, using micro optics and finally this beam can be coupled into 125µm fiber with core diameter of 50 µm. While the optical power meter system (PM213) provided by THORLABS with two different detectors; (S120B) silicon sensor has spectral band ranging from 400:1100nm measuring an optical power ranging form 50nW-50mW and the other (S213A) Thermal sensor has spectral band ranging from 250:1064nm measuring up to 30W. Using the silicon sensor for low power measurement since its response is a wavelength dependent.

b. Measurements results: By adjusting the laser diode driver to different voltages and obtaining the driving input current to the diode module (measured and viewed through the computer’s screen) at different operating temperature (10, 15, 25, 35 and 45oC); the measured (I-V) diode characteristic curve is shown in Fig. 2. From Fig. 2 one can notice that; the maximum consumption electrical power was found at temperature of 10oC provided the lowest electrical to optical slope efficiency in this case. I-V Characteristics for the 976 nm Diode Laser 6 45° C 35° C 25° C 15° C 10° C

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Paper: ASAT-13-TE-23

c. Optical characterization of the fiber coupled Diode module: The experimental setup was shown in previous section.

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Principles and concepts of the measurements:

The light emitted by one facet of the semiconductor (Diode) laser was measured as a function of the driver input current (I); and the measuring curve is referred to as the Light-Current (LI) curve which is strongly temperature dependent. The form of the (L-I) curve is typical the same as in any lasers; the turning point at which the laser output abruptly start to increase corresponds to the threshold lasing point. The threshold current (Ith) or equivalently threshold current density (Jth) is an important device parameter and its minimization is desirable. It is well known that when the input current I < Ith; light output mainly consists of spontaneous emission.

ii- Optical characterization measurements results:

The output laser power at a constant operating temperature (25oC) is measured using the optical power meter and the laser detectors; corresponding to the change in the driving input current to the Diode laser module. As the optical characteristics (L-I) curve can be investigated. Diode output laser power (Watt) Vs. the diode input driving current (A); measured at different temperatures 10, 15, 25, 35 and 45oC. Output measured threshold laser was 528 µW at input driving current of 0.3A (using S120B photo detector for measuring such low output power). The (L-I) characteristic curve is measured at different temperatures showing the temperature effect on the threshold lasing point and the slope efficiency shown in the next section. L-I Characteristics of 976 nm Laser Diode 45°C 35°C 25°C 15°C 10°C

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Fig. 3 The Optical characteristics (L-I) curve of the high power diode module at operating temperature of 10, 15, 25, 35 and 45oC 4/10

Paper: ASAT-13-TE-23

Figure 3 shows the clearly linear relation between the output laser power and the input driving current behind threshold point, as the laser is shown to operate continuously without any indication of heating or failure. We studied the conversion efficiency of the pigtail fiber by measuring the output optical power from the pigtail fiber coupled diode module for the driving current 4A that was 3.35W; i.e. we have conversion efficiency around 83.75% ; which is considered as high conversion efficiency between the Diode module and the fiber pigtail. From the previous Electrical and Optical characterization measurements; we got the important relation between the output optical power and the input electrical one shown in Fig. 4. Output Optical Power vs Input Electrical Power 45 °C 35 °C 25 °C 15 °C 10 °C

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Figure 4 shows the relation between the optical output power and the electrical input one in the above curve; while the lower one shows the electrical to optical slope efficiency. From our measurements shown in Fig. 4; we have maximum slope efficiency around 36%. The decreasing in the measured efficiency with increasing pumping current shown in Fig. 4 occurs because of the ohmic losses in the diode laser increasing with the square of the current [1]; where the output power is in linear relation with the driving current as shown in Fig. 3.

3. The Temperature effects on the Output diode laser characteristics a. Temperature effect on the lasing threshold point: Using the previous experimental setup; with changing the diode operating temperature to be 10, 15, 25, 35 and 45oC; and measuring the (L-I) curve in each case, the temperature effect on the output threshold laser can be examined.

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Fig. 5 The laser diode output vs. the driving input current at temperatures (10, 15, 25, 35 and 45oC) Figure 5 shows the laser diode output vs. the driving input current at temperatures (10, 15, 25, 35 and 45oC), the result illustrates that; the laser threshold output is an increasing function of temperature. From these experimental results; it is obviously clear that at temperature of 45oC the (L-I) curve has the highest threshold value; with the lowest conversion electrical to optical slope efficiency in this case. Concerning with the temperature effect on both of threshold lasing and the conversion slope efficiency; and taking into account the measuring results of the electrical characterization shown in Fig. 2; we notice that; operating the high power Diode module at the room temperature (25-30oC) is the best condition. It gives higher slope efficiency with lower consumption electrical power and lower threshold lasing which considered to be an optimum condition producing a maximum output performance. 6/10

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b. Temperature effects on the Wavelength measurements: Temperature variation is an important factor that must be monitor if the wavelength will change with the temperature variation or not. Here we notice that the wavelength will not change that is a measure of the stability of the diode laser regardless of the temperature variation.

i. Experimental setup: The experimental setup shown in Fig. 6 consists of; high resolution Spectrometer, fiber optic integrating spheres, collimated optics, optical density lens, high power pigtail diode module and "Spectra Suite" computer software. The high resolution Spectrometer (HR4000CG) with a 3648-element linear-array CCD detector that provides better optical resolution produced by "Ocean Optics Inc." shown in Fig. 11; providing 200:1100 nm wavelengths ranging with 0.75nm optical resolution (FWHM) is used in our experimental work. "Spectra Suite" spectrometer software is used in analyzing and viewing our measurements.

ii. Experimental results: Firstly; we investigate the temperature effect on the laser output wavelength; we measure the diode wavelength at different operating temperatures controlled by the laser diode driver and temperature controller (within the diode operating temperature acceptance 0:60oC) with a constant driving current 4000 mA. Figure 7 shows the wavelength of the laser output at different temperatures (from 25 up to 35oC with step 1 oC) at constant driving current (4A). We notice that the emitted laser wavelength is not affected by the temperature increasing or by the current increment, we did that to investigate the stability of the unit used. From Fig. 7 we measured the spectrum of the high power diode laser for the operating current of 4A with different temperatures (from 25oC to 35oC with step temperature of 1 oC) we notice no shift in the output wavelength at 977.34 nm. Secondly; we measure the wavelength of the diode o/p at different i/p driving currents. Figure 8 shows the output wavelength measured at different driving current (from 1A to 7A with step current 0.5A) for a constant temperature. We repeated this measurement 3 times for 3 different temperatures ( 25oC, 30oC, 35oC). From our measurements; one can observe that the peak wavelengths for the diode laser are nearly constant for different temperatures at the same driving current); with no shift in the central wavelength. The final step in the measurement is the thermal image of the optical beam which is in the shape of a Gaussian beam and the figure below shows the beam profile of the beam. This measurement was done using the FLUKE Ti20 TERMAL IMAGER as in Fig. 9.

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Fig. 6 The experimental setup of the wavelength measuring and at the right the spectrometer ranging from 200:1100 nm is shown

Spectrum Of the 976 nm high power diode laser 4000 35 34 33 32 31 30 29 28 27 26 25

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Paper: ASAT-13-TE-23 Spectrum Analysis for the diode laser at 25°C 6000 7A 6.5 A 6A 5.5 A 5A 4.5 A 4A 3.5 A 3A 2.5 A 2A 1.5 A 1A

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Fig. 8 The spectrum of the 976 nm high power diode laser operating at current range from 1A to 7A with step current 0.5A for different temperatures a) 25oC, b)30oC, c)35oC 9/10

Paper: ASAT-13-TE-23

Fig. 9 Output beam profile

4. Conclusion: In this paper; the temperature effect on both of threshold lasing and the conversion slope efficiency was measured. Based on the Electrical and Optical characterization measuring results; operation of the high power diode module at the room temperature (25oC) gave best slope efficiency with low consumption electrical power and low in lasing threshold; which is considered as an optimum condition that gives the maximum output performance. Maximum slope efficiency around 35.84% with a maximum electrical to optical conversion efficiency of 25% was measured. The emitted laser wavelength was not affected by the temperature increasing. Moreover; the appropriate wavelength that useful for pumping Yb doped fiber (around 975nm) was obtained at the temperature range of (25oC). The peak wavelengths for the diode laser were found to be nearly constant for different operating temperatures. Typical 976 nm diode laser o/p was obtained with different driving i/p current at constant temperature of 25oC with line width of 1.3 nm at FWHM.

5. References [1] [2] [3] [4] [5]

W.Streifer, D.R.Scifres, G.L.Harnagel, D.F.Welch, J.Berger and M. Sakamoto "Advances in Diode Laser pumps" IEEE, Q.E vol24.1988. O.Svelto ”Principles of Lasers” fourth edition, page 201-224. W.Koechner ”Solid State Laser” fifth edition,1999,Page 47-63,134-147,463-469. R.Paschotta , www.rp.photonics.com, © 2007. C.A.Wang and S.H.Groves "New Materials for Diode Pumping Solid State Lasers" IEEE, Q.E.vol28.1992. 10/10

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