Models 61XX User’s Manual

The External Cavity Tunable Diode Laser (Patent Pending)

CAUTION: Use of controls or adjustments, or performance of procedures other than those specified herein, may result in hazardous radiation exposure.

Warranty New Focus, Inc. guarantees its lasers to be free of defects for three months from the date of shipment, or for 3000 hours of operation, whichever comes first. This is in lieu of all other guarantees, expressed or implied, and does not cover incidental or consequential loss.

Contents

612656 Rev. F

2

Warranty

2

User Safety

3

Safety Warning Labels

4

Required Items

6

Introduction

7

Theory

8

Operation

13

Appendix 1: AC Operating Voltages

21

Appendix 2: Maintenance

23

Specifications

24

Is a registered trademark of New Focus Inc.

User Safety

Your safe and effective use of this product is of utmost importance to New Focus. Please read through the entire manual before attempting to operate the laser. 3

Laser Safety Warnings

Model

Max Power

Wavelength Range

6102

25 mW

635–700 nm

6124

50 mW

700–820 nm

6126

60 mW

820 nm–1.06 µm

6148

20 mW

1.2–1.4 µm

6162

20 mW

1.4–1.6 µm

Table of maximum internal powers and possible wavelength ranges by model number.

The laser light emitted from this unit may be harmful to the human eye. Avoid looking at the laser beam directly. The following safety labels are attached to this product as described on page 5. Note: The user will NEVER need to open the laser head. Contact New Focus if, for some reason, you want to open the laser head. Unauthorized opening of the laser head will void the warranty, and may result in misalignment of the laser cavity and/or irreparable damage to the internal components. Diode laser power at the wavelength shown in the table above could be accessible in the interior of the laser head. 4

ID/Certification Label Located on rear of controller. Model Number: Serial Number: Manufactured: NEW FOCUS, Inc. 1275 Reamwood Ave. • Sunnyvale, CA • 94089-2256 This product conforms to the applicable requirements of 21 CFR 1040.10 and 1040.11 at the date of manufacture.

Warning Label Located on side of laser head.

DANGER INVISIBLE LASER RADIATION AVOID DIRECT EYE EXPOSURE Power Output See Table pg. 4 Wavelength See Table pg. 4 CLASS IIIb LASER PRODUCT

Aperture Label

Protective Housing Label

Located at output aperture of laser head.

Located inside laser head. DANGER Laser Radiation When Open AVOID DIRECT EYE EXPOSURE

INVISIBLE LASER RADIATION EMITTED FROM THIS APERTURE

AVOID EXPOSURE

5

Required Items

The following items are needed for a 61XX New Focus laser system: 6100 Laser Controller 61XX Laser Head Laser cable (between controller and head) AC power cable Before connecting the AC power cable, verify that the line voltage setting in the laser controller agrees with the local voltage. (See Appendix 1: AC Operating Voltages, pg. 21.) Note: Substitution of components other than New Focus products could result in damage to either the laser head or the controller. 6

Introduction

Tunable diode lasers, such as the New Focus Model 61XX external-cavity diode lasers, are useful in applications such as atomic absorption, Raman spectroscopy, cooled ion and atom experiments, and vapor-rate monitoring systems. The Model 61XX diode laser provides truly continuous tuning with less than 100 kHz of linewidth. Through a mechanical adjustment on the laser head, you can achieve coarse control of the frequency to within 2 GHz. Fine adjustment to within 100 kHz is done with piezo actuator motion accessed from the Model 6100 controller front panel. The external cavity diode laser includes both the laser head and the diode controller. The laser head houses the diode laser, grating, tuning mirror, and collimation optics (see Fig. 2, pg. 9). The external grating acts as both the wavelength-selective device and the output coupler. The diode controller includes the temperature, current, and diffractionangle control boards. 7

Theory External-cavity diode lasers achieve their superior stability through several factors: temperature and current control of the diode, diffraction-angle control, and environmental isolation of the head. The Model 6100 laser controller can be used with any New Focus laser head and includes the temperature, current, and diffraction-angle control boards. (In some instances, the installation of an additional card in the diode controller will be required.) Fig. 1 is a block diagram of the Model 61XX tunable laser system. The New Focus external-cavity diode lasers consist of a diode laser, diffraction grating, and retromirror in a Littman-Metcalf configuration as shown in Fig. 2. The diffraction grating acts as the frequency-selective output coupler. The grating is highly dispersive and reflects a very narrow spectrum of radiation back into the laser. This, along with the extended cavity length, results in a laser that oscillates in a single longitudinal mode with a very narrow linewidth and broad tunability. The tunability of the external-cavity diode laser is achieved through angle control of the retro-mirror. Coarse tuning of the angle is actuated with an 80-pitch screw and fine tuning is achieved through the use of a PZT stack mounted directly on the screw. The output power as a function of wavelength (angle) is shown in Fig. 3 (pg. 11) for a typical diode laser. The frequency stability of the laser is dramatically improved over that of a free running diode. This is attributed to the use of the grating, together with the longer photon lifetime associated with the increased cavity length. The linewidth of the laser 8

Fig. 1 Block diagram of the Model 61XX. Diffraction Angle and Alignment Control

Diode Laser

Diode Current Control

Modulation to 1 MHz

Diode Temperature Control

Diffraction Angle Control

5-mK Stability

>30-GHz Tuning 200-Hz Bandwidth

Output

Fig. 2 Laser head external-cavity arrangement.

Heat Sink Tuning Mirror

PZT

HR Laser Diode

Output Beam Lens

Temp. C

ont.

Current C

ont.

AR Coating

Diffraction Grating

Control In

9

Theory cont.

is shown in Fig. 4. This data is obtained by mixing two nominally identical lasers on a photodiode and observing the frequency stability of the generated RF frequency difference. Temperature stability of the diode laser is better than ±5 mK. The current controller has a noise of less than 100 nArms in a 1-MHz bandwidth with a 0–100 mA range available. Precise temperature control is achieved by independently controlling the temperature of the laser diode and its environment; see Fig. 5 (pg. 12) for a block diagram. Front-panel current modulation to 1 MHz with minimal impact on the frequency noise is possible. If the laser power is externally monitored and the corresponding electrical signal is fed back into the controller’s current modulation input, the laser can operate in a constant output power mode. The laser power is monitored at the back facet of the laser diode. Power is controlled at the front panel of the controller box. The panel meter reads actual power in milliwatts to within ±20%; if a more accurate reading of the output power is needed, use a power meter. 10

Fig. 3 Typical output power as a function of wavelength.

Power (mW min. and max.)

20

15

10

5

0 840

845

850

855

860

865

870

Wavelength (nm) This example data is for a Model 6126 laser. Please see the back of this manual for actual data for your model.

Fig. 4 Typical linewidths for a duration of 5 s and 50 ms.

5 dB/div

Resolution Bandwidth 2.0 MHz Averaging Time 5 s

Resolution Bandwidth 100 kHz Averaging Time 50 ms

5 dB/div

10 MHz/div

50 MHz/div

11

Fig. 5 Temperature control block diagram.

Laser Head

Diode T sense (from head)

-

T set

G

TEC

G

TEC

+ T sense (from head)

-

T set +

Environment

12

Operation This section outlines the instructions for normal use of the tunable diode laser. Please refer to Figs. 6 and 7 (pg. 14), which show the front and rear panels of the 6100 controller. NEVER look into the laser beam. Always view the beam on a diffuse surface such as a business card or IR viewing card. Avoid creation of random reflections, scatter, or stray beams that could be dangerous. Power-Up Sequence 1. Mount the laser head at the desired location for operation using the tapped mounting holes in the bottom. Ensure that the head is stable. 2. Place the controller box within reach of the head using the five-foot cable provided. 3. Connect the head to the controller with the cable. Each end of the cable is keyed; one end has a single tab and one end has two tabs. The connector with one tab mates with the head. 4. Verify that the line voltage setting in the laser controller agrees with local voltage. (See Appendix 1: AC Operating Voltages, pg. 21.) Plug the AC power cord from the controller into an outlet. Note: Laser power will not go on until the key lock is turned and the power button is pushed. 5. If the interlock is used, connect it to the terminals on the rear of the controller. If it is not to be used, ensure the jumper wire shorting the terminals is in place. 6. Turn the keyed lock to the “1” position. Note 13

Fig. 6 Model 6100 controller front panel.

External Cavity Tunable Diode Laser 6100

Made in U.S.A.

Laser Power

Fine Frequency

Current (mA) Temperature (°C)

Power (mW)

0

Read 1

AC Power

Frequency Modulation Input

Current Modulation Input

Fig. 7 Model 6100 controller rear panel.

Interlock

Laser

14

Set NEW FOCUS, Inc.

Operation cont.

that the key cannot be removed while it is in the “1” position. The red LED light will illuminate indicating that AC power has been applied. Note: Allow the controller to stabilize for at least 15 minutes before proceeding. 7. Adjust the laser-diode temperature. Note: The diode temperature is already optimized and doesn’t generally need resetting. Please refer to the graph included in the pocket at the end of this manual for the operating temperature range. a. Hold the front panel switch in the “Set” position. b. Set the temperature with the adjustment knob while monitoring the panel meter. c. Release switch to view actual temperature. Depending on the magnitude of the change in the set temperature, it may take up to two minutes for the actual temperature to converge on the set temperature. If the actual temperature does not reach the set temperature, contact New Focus. DO NOT turn on the laser diode. 8. Since the 6100 controller can be configured to drive laser heads at several different wavelengths, the polarity of the current controller varies depending on the head model number. For example, when using the Model 6126 head, the current must be negative as displayed by the “Current” panel meter on the Model 6100 front panel. Without turning on the “Laser Power” button, turn the “Laser Power” knob and ensure the current is negative. If not, the current controller in the Model 6100 must be replaced with one of the opposite polarity. 15

Operation cont. 9. Depress the “Laser Power” button to enable the laser. Note: a green LED light inset in the “Laser Power” pushbutton will illuminate, indicating the laser is enabled; the actual laser power will be applied 3–5 seconds later. Subsequent pressing of the button will toggle the laser power on and off, with 3–5 seconds of delay each time the laser power is turned on. 10. Set the laser power. a. Set the power with the “Power” knob while monitoring the panel meters for power and current. b. Note that the current meter will change as adjustments are made to the power. The optimum current setting is provided in the data included in the back of this manual (threshold data and recommended operating current are provided on the test data sheets). 11. Select the desired wavelength by using the coarse adjustment on the laser head. Note: You must remove the protective setscrew in the coarse adjustment access hole before you can access the coarse adjustment screw. This protective setscrew is there to prevent accidental coarse adjustment of the laser. The coarse knob setting is accurate to approximately 1 Å. Whenever possible, a monochrometer or wavemeter should be used. Fine frequency control of the wavelength can be realized with 16

Operation cont.

the “Fine Frequency” potentiometer on the front panel of the laser controller. This actuator will provide ±0.5 Å of tuning resolution and the effect will only be noticeable with a high resolution wavemeter or monochrometer. Note: Once the wavelength is set the output power can be optimized with modifications of the temperature of ±0.5˚ C and/or the current. At any given wavelength or current, a set of optimum temperatures exist that will maximize the power. With the wavelength of the laser fixed, a small adjustment (~0.1 °C) of the temperature set point will allow peak power to be achieved. Power-Down Sequence Note: All of the laser settings can be preserved and the temperature stability of the head maintained by switching off only the laser power using the “Laser Power” pushbutton on the front panel. This will increase the lifetime of the diode without requiring long set-up times due to the time required for the temperature to stabilize. If, however, you wish to power down everything, including the temperature controllers, follow these steps: 1. Bring laser power completely down using the “Laser Power” knob on the front panel. 2. Turn the AC key switch to the “0,” or off, position. 17

External 1. Current Modulation Modulation Connect the modulating source to the 50-Ω inputimpedance, current-modulation, BNC connector on the front panel. Since the input signal is dropped across 5 kΩ, an input voltage of 5 V will give a current modulation of 5 V/5kΩ = 1 mA. Smaller or larger voltages can be used to modulate the current, but to avoid damaging the input amplifier, amplitudes should be kept less than 10 V peak. Monitoring the output power of the laser with a photodiode and applying negative feedback to this input will allow operation of the laser in a constant-power mode. This is useful in some applications where amplitude stability is more important than frequency stability. 2. Frequency Modulation Connect the modulating source to the “Frequency Modulation” input BNC connector on the front panel (which has an input impedance of approximately 6 kΩ). Since the high-voltage amplifier driven by the frequency-modulation input has a gain of approximately 22, a ±3 V signal on the input will swing the voltage applied to the piezo ±66 V, which yields a tuning range of at least 60 GHz. Input signals at rates exceeding 200 Hz will be current limited and will not effectively modulate the frequency. Since the input is protected with limiting diodes, signals with amplitudes greater than 3 V will not damage the unit. The protection circuitry limits the voltage applied to the piezo actuator in the laser head. Because the voltage applied to the piezo is the sum of the DC voltage derived from the “Fine Frequency” front-panel knob and the “Frequency 18

External Modulation” input, the knob should be set to the Modulation middle of its range, or 5, if external modulation cont. over the full 60-GHz range is desired. Otherwise, if

a DC offset is applied via the knob, the controller protection circuitry will limit the voltage applied to the piezo and this will limit the wavelength. Modulation over a range less than 60 GHz is achieved simply by reducing the input drive voltage below 3 V. A DC offset can then be applied via the front-panel “Fine Frequency” knob to obtain modulation around a wavelength different from that obtained with the knob in its center position. There are two techniques to ensure that the voltage applied to the piezo is not limited by the protection circuitry for a particular combination of input frequency modulation signal and DC offset from the “Fine Frequency” knob. The first is to monitor the voltage on the output connector labeled “Laser” on the rear panel while the electric input is applied to the front panel. The procedure is as follows: a. Power down the laser controller as per the instructions appearing on page 17. b. Remove the cable from the “Laser” connector on the Model 6100 controller. c. Monitor with an oscilloscope the piezo voltage on pin 13 of the “Laser” connector and adjust the frequency modulation input amplitude and “Fine Frequency” knob until the piezo voltage is not limited, or clipped. See Fig. 8 (pg. 22) for a pin-out of the connector. d. Power down the laser controller. e. Re-connect the cable. 19

External Modulation cont. f. Power up the controller. If it is not desirable to power down the laser to ensure that the frequency of the laser is not limited during modulation, an optical technique may be used. Simply monitor the wavelength of the laser with an optical spectrum analyzer or wavemeter. During a low frequency scan, reduce the modulation depth or adjust the DC offset until the scan is smooth and nonclipped. If a specific modulation range is required, the only option is to adjust the DC offset until clipping is avoided. The maximum scan range is ~2 cm-1 and can only be achieved with the DC offset set to the middle of its range (5 on the “Fine Frequency” scale).

Interlock A remote interlock is included on the back panel of the laser diode controller to comply with safety requirements. In order for the current driver to operate normally, this must be shorted. To use the interlock feature, remove the jumper and replace it with a circuit (i.e., a door switch) which will be shorted in the normal position. Once the connector is opened, the current driver will no longer operate. 20

Appendix 1: AC Operating The Model 6100 Controller can operate from 100, Voltages 120, 220, or 240 V AC at AC frequencies of 47-63 Hz. The unit is configured at the factory for the standard AC voltage in the owner’s country. To select a different operating voltage, please refer to Fig. 9 (pg. 22) and follow the directions below: 1. Disconnect the power cord. 2. Open the cover of the power entry module on the rear panel with a small-blade screwdriver or similar tool. 3. Insert the tool into the voltage selection slot and remove the wheel from the unit. 4. Turn the wheel and re-insert into the module so the desired voltage is shown. Do not attempt to rotate the wheel while it is still in the power entry module; the wheel must be removed, turned, and then re-inserted. 5. Close the cover. 6. Verify that the proper voltage is showing through the module’s window. The power entry module requires two 5x20 mm, slow blow fuses, such as Littlefuse’s® Slo-Blo® 239 series: one for the hot line and the other for the neutral line. Replacement fuses should be as follows: AC Voltage 100 V AC 120 V AC 220 V AC 240 V AC

Fuse Rating 2.0 A 2.0 A 1.6 A 1.6 A

Little Fuse 239 002 239 002 239 01.6 239 01.6 21

Fig. 8 Laser connector pin out.

Pin 1

Pin 12

Pin 13

Pin 18

Fig. 9 Model 6100 voltage selection.

22

Appendix 2: Maintenance

Virtually no maintenance is required for this product, with the exception of ensuring that the unit is not damaged, contaminated, or used in an unsafe manner. Any questions regarding operation or performance should be directed to New Focus engineers. If maintenance or repair is required, the unit should be shipped to New Focus. 23

Specifications

24

Center Wavelength:

_____________

Tuning Range:

_____________

Fine Frequency Range:

_____________

Power:

_____________

Wavelength Stability:

0.01 nm/day

Repeatability Error:

2 GHz (max)

Linewidth (50 ms):

100 kHz (max)

Linewidth (5 s):

5 MHz (max)

Sidemode Suppression:

40 dB (min)

Polarization:

Linear (100:1 min)