Schematic Showing a Modified Litman-Metcalf Cavity
Tuning Characteristics
How does New Focus achieve mode-hop-free, single-mode tuning?
All New Focus lasers start out as commercially available semiconductor diode lasers. These diodes typically operate with several longitudinal modes lasing simultaneously, leading to low coherence and large linewidths. One method of extracting highly coherent light from a semiconductor-based laser requires that you anti-reflection (AR) coat the diode so it acts only as a gain element. The diode can then be placed in an external cavity that contains wavelength-selective optics so that only a single mode lases at any given time. True single-mode tuning requires that the optical feedback be dominated by the external optics and not by reflections from the diode facet. We use a proprietary AR-coating process to reduce residual diode reflectivities to below 0.001 which guarantees single-mode operation.
Once the diode is coated, we place it in an external laser cavity that is based on the modified Littman-Metcalf configuration. In this cavity a grazing-incidence diffraction grating and a tuning element provide all the necessary dispersion for single-mode operation the amplitudes of non-lasing modes are suppressed to 40 dB below the lasing mode.
The wavelength in a modified Littman-Metcalf laser is changed by tilting the retroreflector, which changes the diffracted wavelength fed back into the cavity. To prevent mode hopping, the cavity length must be kept at a constant number of wavelengths as the laser tunes. This requires that the pivot point around which the element tilts be positioned with sub-micron accuracy. Using a patented technique for pivot-point location, we produce lasers with no mode hops. For all of lasers we can guarantee absolutely no mode hops over the entire specified tuning range.
How can I tell if the laser mode hops?
If the laser does mode hop, the frequency change will be 3 GHz, which is equal to the external-cavity free-spectral range. (In contrast, for a poorly coated diode laser, it can be as much as 100 GHz.) We individually test every laser we build and ship them with printouts of their tuning curves, so you'll know exactly what to expect from your laser. (Please contact us if you would like to see sample tuning curves in your wavelength range.)
In what temperature range should I operate my laser to maintain single-mode operation?
The lasers will operate best in a temperature range from 15 to 35 °C, outside of which we can no longer guarantee single-mode operation.
What's the difference between coarse and fine tuning?
For our TLB-6700 Velocity™ Widely Tunable Lasers, we separate wavelength tuning of our lasers into coarse and fine tuning. Coarse tuning is accomplished by using a DC motor to turn a precision screw. An angle sensor incorporated in the laser cavity feeds back to the microprocessor to scan to the desired wavelength. On the end of the coarse-tuning screw is a piezoelectric actuator. This actuator provides independent fine control of the laser wavelength and can be used to modulate the laser frequency while it is being slowly scanned. Our TLB-6900 Vortex™ II Tunable Lasers and StableWave Tunable Lasers use this same fine-tuning piezoelectric mechanism.
How precisely can I set the wavelength?
The coarse tuning mechanism on the Velocity lasers allows you to set the wavelength with a resolution of 0.02 nm. The fine-frequency tuning resolution of the Velocity, Vortex, and Stablewave lasers is 10 MHz or about 2x10-5 nm when using the controller, with a range of approximately 0.2 nm.
How long after adjustment will the piezo become stable and how much drift will there be? (Vortex, Velocity, and Stablewave Series only)
There is roughly a 5% drift on the piezo over a period of several seconds after a significant wavelength change as the piezo relaxes. There should not be any significant amount of shift in wavelength from the wavelength displayed on the screen.
With the Vortex and Stablewave tunable laser, how can I perform high-resolution wavelength scans over a narrow wavelength range?
If you want to perform high-resolution wavelength scans over a narrow wavelength range, use the analog Frequency Modulation input on the back panel of the laser controller. The reason for this is that the electronic gain of the Frequency Modulation input can be selected to be either 25x or 1x. This can only be done with the Vortex laser system, and it requires issuing a command through the GPIB or RS-232 interface. By selecting the 1x gain for the Frequency Modulation input, the frequency resolution of the Vortex will be very high because it is less susceptible to electronic noise pick-up from the analog input signal.
Output Beam Characteristics
What is the output-beam polarization?
The output beams from all New Focus tunable lasers are linearly polarized in a vertical plane. The polarization ratio for our lasers near 633nm is 50:1 and the polarization ratio for all other lasers is 100:1.
What is the output-beam quality?
All of our lasers are nearly diffraction-limited, because they operate in the fundamental transverse mode (because of the high divergence of some diodes, you may see some clipping effects on the output beam). For most of our TLB-6900 Vortex™ II Tunable Lasers, Velocity™ Widely Tunable Lasers and StableWave Tunable Lasers, the beam shape is elliptical, with approximately a 3:1 aspect ratio.
What's the fiber used with the pigtailed lasers?
We use a single mode PM (polarization maintaining) Panda fiber with a cladding diameter of 125 µm and a 3mm jacket. Both ends are terminated with FC/APC connectors with a wide key that is pre-aligned and locked to the slow axis. The coupling loss from the laser to the fiber pigtail is 3 dB. In other words, the typical coupling efficiency into the fiber pigtail is 50%. The PM fiber is aligned with the polarized axis to within two degrees of the output polarization of the laser. This results in an 18 to 20-dB polarization ratio (approximately 100:1).
Frequency Characteristics
What's the frequency stability?
In constant-current mode, the frequency drift of the laser is less than 0.02 nm over an entire day and less than 5 MHz over a one-second interval. Stabilizing the frequency of the laser is easy with an error feedback signal to the Frequency Modulation input on the back panel. Using this technique, wavelength drift can be dramatically reduced.
Can I stabilize the frequency of the laser?
The laser can easily be locked either to an atomic line or to an external cavity using wavelength modulation techniques to produce a feedback signal. Dither the laser frequency through the Frequency Modulation input. The error signal generated can then be fed back to stabilize the laser.
What is the minimum observable frequency shift over >1-second intervals?
The minimum frequency shift that is observable over the frequency jitter of the laser is 1 MHz.
What is the wavelength-modulation bandwidth?
While you can modulate the wavelength by scanning the drive motor (coarse tuning) or by changing the laser-drive current, the most straightforward way to modulate the laser wavelength is by changing the voltage to the PZT actuator on the tuning element. The small-signal bandwidth of the fine-frequency input is 2 kHz in the Velocity lasers and 3.5 kHz in the Vortex and Stablewave lasers.
What happens to the signal after the Frequency Modulation Input?
The voltage that you apply at the Frequency Modulation Input is amplified by an analog high-voltage amplifier, and then applied to the piezo. This circuit does not have a DA converter. Therefore, you should see a smooth fine-frequency tuning response when you apply an analog signal to the Frequency Modulation Input.
Is the minimum frequency resolution different than 10 MHz when using a computer?
If a computer is used to control the fine-frequency tuning of the laser, then the resolution will be approximately 100 MHz or less. The reason for this is that when using a computer, the minimum step size of the piezo voltage is 0.1 volts. A 0.1-volt voltage change on the piezo corresponds to a frequency shift of approximately 100 MHz.
The Velocity, Vortex, and Stablewave lasers have a minimum frequency resolution of 10 MHz (or, 2x10-5 nm). However, the only way to access this minimum frequency resolution is by using the Frequency Modulation input on the back panel of the laser controller. The Frequency Modulation input is an analog voltage input that is used to control the piezo voltage.
So, if you want to perform a wavelength scan with a frequency resolution of 10 MHz, then you will need to supply a low-noise analog voltage signal to the Frequency Modulation input. Alternately, if you want to scan the laser wavelength in 0.001 nm steps (or, approximately 300 MHz frequency steps), then you can use the computer interface to change the piezo voltage in 0.1-V steps.
What is the input impedance and protection circuitry of the frequency modulation input?
The input impedance is 4 k Ω. The input protection circuit consists of two 4-V Zener diodes tied to ground
Coherence Effects
Can I eliminate coherence effects due to the narrow linewidth?
If you're working with unterminated fibers, you may be concerned about possible coherence effects due to the narrow linewidth. For most measurements, a simple way to make the linewidth appear much broader is to apply a signal to the piezo in order to frequency modulate the laser.
Output Intensity Characteristics
How stable is the output amplitude? Can I amplitude-stabilize the laser?
The amplitude stability of our Velocity lasers over 10 seconds is better than 0.25% at any given wavelength. In stable-power mode, the amplitude is typically constant to within ±5% as the laser is tuned. (Stable-power mode is available only for scans at less than 1% of the maximum scanning speed and it is not available at all wavelengths.) To prevent optical feedback from affecting the laser's performance, we recommend using an optical isolator with our free-space lasers. To prevent étalon effects in free space or fiber components, which could create amplitude modulation while scanning, we recommend slightly tilting your optics and using FC/APC fiber connectors.
What limits the intensity noise?
The intensity noise of our ECDLs is directly related to the current noise in the laser-diode drive current. In constant-current mode, the intensity noise of the laser output is limited by the current noise of the drive electronics. Our low-noise controllers make our systems among the quietest ECDLs available at <0.1% RIN. In stable-power mode, the current noise is necessarily increased by the feedback mechanism. This results in small increases in both short-term intensity noise and linewidth.
Can I modulate the laser amplitude? Will the wavelength be affected?
You can modulate the laser current by applying analog voltages to the Current Modulation Input at a rate of up to 1 MHz with a modulation amplitude of up to 2-mA peak. You can also modulate the laser's amplitude with a bandwidth of 100 MHz and an amplitude of up to 10-mA peak through the SMA connector on the laser head. Small changes in the index of refraction of the laser-gain medium as a function of laser current leads to changes in the laser wavelength when the current is modulated. The wavelength-modulation coefficient depends strongly on each laser diode's characteristics, but is typically 50 MHz/mA and at most 1 GHz/mA.
What is the input impedance and conversion of the Current Modulation Input?
The input impedance is 5k Ω and conversion is 0.2mA/V.
Changing Laser Heads
Are the laser heads interchangeable?
Yes. The same controller will work with any laser head in its family. However, for the TLB-6700 Velocity™ Widely Tunable Lasers, wavelength calibration for each laser head is guaranteed only when it is used with its original controller. Laser heads will not work with controllers from other families. (When ordering additional laser heads for a controller, initial calibration of the head to the controller may be required.)
How do you change laser heads to reach different wavelength ranges?
Prealigned optics give you no-fuss interchangeable heads. Positioning the laser-cavity optics for mode-hop-free tuning is a time-consuming task, and the New Focus approach to changing wavelengths takes this burden off your shoulders. We do all the precision adjustments at our factory and send you a sealed, adjustment-free laser head. Should you need another wavelength outside the range of your first laser head, you can simply plug in a different laser head, with no adjustments needed. (When ordering additional Velocity laser heads, initial calibration of the head to the controller may be required.)
How many laser heads can I operate at one time?
Each controller can operate one laser head at a time. To operate two lasers simultaneously, you need two controllers.