Cateye External Cavity Diode Laser Model CEL002 Revision 1.
Limitation of Liability MOG Laboratories Pty Ltd (MOGLabs) does not assume any liability arising out of the use of the information contained within this manual. This document may contain or reference information and products protected by copyrights or patents and does not convey any license under the patent rights of MOGLabs, nor the rights of others.
Preface Diode lasers can be wonderful things: they are efficient, compact, low cost, high power, low noise, tunable, and cover a large range of wavelengths. They can also be obstreperous, sensitive, and temperamental, particularly external cavity diode lasers (ECDLs). With external cavity feedback and advanced electronics such as the MOGLabs DLC external cavity diode laser controller, a simple $10 100 mW AlGaAs diode can become a research-quality narrow-linewidth tunable laser.
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Safety Precautions Safe and effective use of this product is very important. Please read the following laser safety information before attempting to operate the laser. Also please note several specific and unusual cautionary notes before using MOGLabs lasers, in addition to the safety precautions that are standard for any electronic equipment or for laser-related instrumentation.
iv the keyswitch is in the STANDBY position. The key cannot be removed from the controller when it is in the clockwise (RUN) position. • To completely shut off power to the unit, turn the keyswitch anti-clockwise (STANDBY position), switch the mains power switch at rear of unit to OFF, and unplug the unit. • When the STANDBY/RUN keyswitch is on STANDBY, there cannot be power to the laser diode, but power is still being supplied to the laser head for temperature control.
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vi Model number: ECD-004 Serial number: A42034011208-01 Manufactured: JULY 2012 Complies with 21 CFR 1040.10, and 1040.11 except for deviations pursuant to Laser Notice No.
Protection Features MOGLabs lasers includes a number of features to protect you and your laser. Protection relay When the power is off, or if the laser is off, the laser diode is shorted via a normally-closed solid-state relay at the laser head board. Emission indicator The MOGLabs controller will illuminate the emission warning indicator LED immediately when the laser is switched on. There will then be a delay of at least 2 seconds before actual laser emission.
RoHS Certification of Conformance MOG Laboratories Pty Ltd certifies that the MOGLabs External Cavity Diode Laser does not fall under the scope defined in RoHS Directive 2002/95/EC, and is not subject to compliance, in accordance with DIRECTIVE 2002/95/EC Out of Scope; Electronics related; Intended application is for Monitoring and Control or Medical Instrumentation. MOG Laboratories Pty Ltd makes no claims or inferences of the compliance status of its products if used other than for their intended purpose.
Contents Preface i Safety iii Protection Features vii RoHS Certification of Conformance viii 1 Introduction 1.1 External cavity . . . . . . . . . . 1.2 Mode competition . . . . . . . . . 1.3 Piezo-electric frequency control 1.4 Temperature and current . . . . . . . . . 1 2 3 3 4 2 First light 2.1 Temperature . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Current . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contents x B Connector pinouts B.1 Headboard . . . . . . . . . . . . . . . . . . . . . . . . . B.2 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.3 RF coupling . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Semiconductor laser diodes are compact, efficient and low-cost, but usually have poor wavelength control, linewidth and stability. The addition of an external frequency-selective cavity allows control of the operating wavelength over a few nm range, with sub-MHz linewidth and stability. The MOGLabs CEL (see Fig. 1.1) is machined from a solid aluminium block, so that the laser is stable, robust, and insensitive to acoustic disturbances.
Chapter 1. Introduction 2 is used to select a single external cavity mode. Without the need for illuminating a large area of a grating for feedback, a cat-eye retroreflector and partially transmitting output coupler can be used to form the external cavity. The cateye reflector is inherently selfaligning, so that the laser is extremely insensitive to mechanical disturbance, and also ensures high feedback coupling efficiency and consequently narrow linewidth.
1.2 Mode competition 3 ternal cavity determines the lasing wavelength. The external cavity is typically around 40 mm long from rear facet of semiconductor to output coupler, giving a cavity mode spacing (FSR) of c/2L = 3 to 4 GHz. The laser diode and collimating lens are held rigidly in a focusing tube. The filter is fixed to a bearing-mounted rotation assembly with fine actuator screws to adjust the angle.
Chapter 1. Introduction 4 Diode cavity Diode gain External cavity Filter COMBINED 384.0 384.1 384.2 Frequency (THz) 384.3 384.4 Figure 1.3: Schematic representation for the various frequency-dependent factors of an ECDL, adapted from Ref. [4], for wavelength λ = 780 nm and external cavity length Lext = 15 mm. 1.4 Temperature and current The laser frequency is also dependent on temperature and injection current; the sensitivities are typically 3 MHz/µA and 30 GHz/K [5].
2. First light Initial installation of the laser is typically a matter of mounting it to an optical table and connecting to a MOGLabs controller. The laser can be mounted to posts using the M3 threaded holes on the base, or by removing the cover and screwing directly to the optical table using the M6x25 socket head cap screws provided. The hole spacing also allows direct mounting to imperial tables for non-metric countries (Burma, Liberia and the USA).
Chapter 2. First light 6 It is assumed that a MOGLabs DLC controller will be used to drive the laser. If an alternative supply is used, note that +5 V must be provided on pin 15 of the headboard connector to open the protective relay. See section B.1 for connection details. Also please refer to the laser test data for the maximum safe operating current. 2.1 Temperature The preferred diode temperature will depend on the diode, the required wavelength, and the ambient room temperature.
2.2 Current Current The output of semiconductor laser diodes follow a nominally linear power vs. current relationship, once the current is above a devicespecific threshold (see Fig. 2.1). Initially the current should be set above threshold, but well below the nominal maximum operating current, until the laser is fully aligned. 140 Bare 150mW diode 780.243nm Extracavity estimate 120 Power (mW) 2.2 7 100 80 60 40 20 0 0 20 40 60 80 100 120 Injection current (mA) 140 160 180 200 Figure 2.
8 Chapter 2.
3. Alignment The cateye reflector arrangement is self-aligning, and should not require adjustment. However, laser diodes have a finite lifetime, and diode replacement may necessitate alignment of the internal optics, in particular the diode collimation and cateye lenses. For longer wavelength lasers, an infra-red upconversion card or CCD camera can be very helpful.
Chapter 3. Alignment 10 Lens Retaining ring 5.6mm diode 9mm diode Figure 3.1: Lens tube assembly, showing diode, lens, and mounting hardware. The same tube can be used for 5.6 mm and 9 mm diodes. Note that your lens tube may have alignment adjustment screws. Figure 3.2: Image showing collimation tubes. does not contact the diode. Also ensure the lens is tight; if not, use PTFE tape on the lens threads. Two layers of thick tape (90 µm as used for gas plumbing) is good. 6.
3.2 Initial diode test 11 distance. 9. Rotate the collimation assembly and adjust the alignment screws until the beam remains reasonably well on-axis. 10. Tighten the retaining ring (hard) and re-check that the diode remains aligned. 11. Focus the collimation lens such that the laser focuses to a spot at some significant distance, more than 4m. The laser stability and modehop free range can be better if the laser output is weakly converging [6]. 3.2 Initial diode test 1.
Chapter 3. Alignment 12 Filter E From diode θ Figure 3.3: Orientation of the diode laser beam ellipse with respect to the filter rotation, for TE polarised diode, oriented with p-plane polarisation. from the filter. In that case, for the MOGLabs CEL, the polarisation should be horizontal and the ellipse should be with long axis vertical. Some diodes, particularly around 750 to 820 nm, are TM polarised, with polarisation parallel to the long axis of the ellipse.
3.4 Cateye reflector 13 35mm Lens tube Cateye Recollimator Figure 3.4: Arrangement of lens tube and cateye reflector for adjustment of focus of cateye. bly at about 35 mm apart, without filter (see Fig. 3.4). Set the diode current just below threshold, and then adjust the cateye lens focus until the output suddenly flashes brightly, indicating effective feedback which tends to lower the overall ECDL gain threshold. Repeat until the minimum is obtained. The sequence is as follows: 1.
14 Chapter 3. Alignment 7. If possible, scan the laser through an atomic resonance and view the absorption on an oscilloscope. With current bias disabled (DIP 4 on a MOGLabs controller) and full span, you should see a reasonable fraction of the absorption spectrum, with one or more mode-hops. A Fabry-Perot etalon or a fast high-resolution wavemeter (MOGLabs MWM) can also be used to optimise the mode-hop-free range. 8.
4. Operation Normal operation of the laser is usually a matter of adjusting the filter rotation angle to select the correct wavelength, and adjusting the piezo offset, diode injection current and bias to achieve the maximum possible mode-hop free scan. 4.1 Wavelength The primary control of wavelength is the filter rotation angle, which can be adjusted while the laser is operational.
Chapter 4. Operation 16 Figure 4.1: Sketch of the MOGLabs CEL, showing the filter rotation adjustment screw, labelled λ. 4.2 Scanning The external cavity length is controlled by piezo actuators moving the output coupler. The cavity length increases with increasing voltage on the piezos, thus decreasing the laser frequency. For a large frequency change, the laser will usually hop to a neighbouring cavity mode (see Fig. 4.2).
4.2 Scanning 17 Relative Gain 1 0 -200 -100 0 100 200 Frequency (GHz) Figure 4.2: Combined gain for an external cavity diode laser, including the internal and external modes, the diode laser gain, and the filter absorption. The predominant feature is the frequency selectivity of the filter, and the smaller peaks are the external cavity modes (see Fig. 1.3).
Chapter 4. Operation 18 4.3 External modulation The laser diode injection current can be modulated directly, or via the SMA RF input on the laser headboard (see section B.3). The combined modulation bandwidth extends from DC to about 2.5 GHz, provided the standard connection from headboard to diode is replaced with a suitable coaxial cable. Even higher frequencies can be used with addition of an appropriate microwave bias-tee such as the Minicircuits ZFBT-6G+, between the laser headboard and the diode.
A. Specifications Parameter Specification Wavelength/frequency 50 mW standard. Up to 150 mW output 780 nm power available. Please contact MOGLabs for availability. 369.5 – 1120 nm Linewidth Typically < 150 kHz RF modulation 160 kHz – 2.5 GHz Filter 0.
Appendix A. Specifications 20 Parameter Specification Thermal TEC ±14.5 V 3.3 A Q = 23 W standard Sensor NTC 10 kΩ standard; AD590, 592 optional Stability at base ±1 mK (controller dependent) Cooling Optional: 4 mm diam quick-fit water cooling connections Electronics Indicator Diode short-circuit relay; cover interlock connection; reverse diode Laser ON/OFF (LED) input 160 kHz – 2.
A.1 RF response A.1 RF 21 response The laser includes an RF bias tee, with typical frequency response shown below. By default, the connection to the laser diode does not provide the full bandwidth of the bias tee. A small circuit board, for RF connection to the diode, is available from MOGLabs; please contact us for further details if required. Ref -20 dBm TG * Att -30 dBm 50 dB * RBW 30 kHz * VBW 10 MHz SWT 17 s -20 -25 -30 -35 -40 -45 -50 -55 -60 -65 -70 Center 1.
22 Appendix A.
B. Connector pinouts B.1 Headboard The laser head interface board provides connection breakout to the laser diode, TEC, sensor, piezo actuators, and laser head interlock. It also includes a solid-state protection relay and passive protection filters, a laser-on LED indicator, and an SMA connection for direct diode current modulation. The connections are made with Hirose DF59 “swing-lock” wire-to-board connectors.
Chassis Earth Mount Hole Single 5 Single 6 Single 3 Single 4 Single 1 Single 2 Pair 6 Pair 6 P6 Shield P0/5 Shield Pair 5 Pair 5 Pair 0 Pair 0 P1/3 Shield Pair 3 Pair 3 Pair 1 Pair 1 P2/4 Shield Pair 4 Pair 4 Pair 2 Pair 2 Laser Laser + 14 15 Relay Relay + 4k99 R2 Active sensor Active sensor + 6 7 8 16 Shield Thermistor + Thermistor Shield Stack Piezo + Stack Piezo - 20 21 19 Disc Piezo + Disc Piezo - 17 18 24 23 22 Shield 11 12 13 Shield 3 Peltier Peltier + 9 10 4 5 1 2
B.2 Laser B.2 25 Laser WARNING: The LASER connector is a standard DVI-D Dual Link socket as used for consumer digital display devices. It should only be connected to the corresponding MOGLabs DLC controller. It supplies the high-voltage signals to drive the laser piezoelectric actuators. The piezo drivers will be disabled if the cable is disconnected, but nevertheless considerable care should be taken to ensure that nonMOGLabs devices are not connected via this connector.
26 Appendix B. Connector pinouts cutoff. For higher bandwidths, use an external bias-tee such as the Minicircuits ZFBT-6G+ between the head board and the diode. The input sensitivity depends on the diode impedance, typically about 50 Ω. Thus a 0 dBm signal corresponds to about 0.2 V and a current of around 4 mA at the diode. That is, the current sensitivity is approximately 20 mA/V. WARNING: The RF input is a direct connection to the laser diode. Excessive power can destroy the diode.
Bibliography [1] Daniel J. Thompson and Robert E. Scholten. Narrow linewidth tunable external cavity diode laser using wide bandwidth filter. Review of Scientific Instruments, 83(2):–, 2012. 1, 2 [2] X. Baillard, A. Gauguet, S. Bize, P. Lemonde, Ph. Laurent, A. Clairon, and P. Rosenbusch. Interference-filter-stabilized external-cavity diode lasers. Opt. Communic., 266:609, 2006. 1, 2 [3] M. Gilowski, Ch. Schubert, M. Zaiser, W. Herr, T. WÃijbbena, T. Wendrich, T. MÃijller, E.M. Rasel, and W. Ertmer.
[9] G. C. Bjorklund. Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions. Opt. Lett., 5:15, 1980. 18 [10] R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward. Laser phase and frequency stabilization using an optical resonator. Appl. Phys. B, 31:97– 105, 1983. 18 [11] M. Zhu and J. L. Hall. Stabilization of optical phase/frequency of a laser system: application to a commercial dye laser with an external stabilizer. J. Opt.
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