TOPTICA’s iScan quadrature interferometer for laser mode monitoring and frequency control.

iScan – Mode and Frequency Control

For laser mode monitoring and frequency control
Fast and precise scanning of tunable lasers
Scan linearization, stabilization to any wavelength
Static and dynamic mode surveillance
Patented interferometer design (US 6,178,002)

The iScan comprises a patented interferometer for fast and precise frequency control of tunable lasers. The laser frequency can be scanned in a precisely linear or step-wise manner. The laser can also be stabilized to any value within its tuning range, including “off-resonant” values where no atomic or molecular transition is available. Applications include laser cooling, interferometry, LIDAR seeding, and cw terahertz experiments.

Technical Information

Concept	Quadrature signal generation in a low-finesse etalon. Stand-alone interferometer head + 19” control electronics.
Input wavelength range	400 – 1100 nm (standard version) 1100 – 1700 nm (IR version)
Input coupling	SM/PM fiber
Input power	Min. 20 .. 100 µW, wavelength dependent
Interferometer free spectral range	8 GHz typ., other FSRs on request
Scan linearity (absolute)	100 MHz typ., higher linearity on request
Scan resolution (relative)	1 MHz typ., higher resolution on request
Scan speed (typ.)	50 GHz / s (DFB lasers)
Long-term stability	< 20 MHz @ 3 hrs, laboratory environment
Error signal bandwidth	Up to 1 MHz
Arbitrary wavelength locking	Possible, resolution typ. 1 MHz, no grid
External control	PC, USB 2.0 interface
Software	Visual Basic sample program included
Operating temperature	15 – 30° C
Electronics	19” control rack
Operating voltage	100..120 V / 220..240 V AC, 50..60 Hz (auto detect)
Suitability	Diode / fiber / dye / solid state lasers

Design of the optical interferometer within the iScan head. A wedge-shaped beam splitter in the head generates two low intensity probe beams (PBA and PBB), which enter the etalon under slightly different angles. The etalon produces a pair of interference signals with a relative phase of π/4 (90°). These signals are detected by two photodiodes (a and b) and combined into a quadrature signal, the phase of which is a linear function of the optical frequency. Two additional photodiodes (Ia and Ib) behind the etalon provide normalization values.

Quadrature signals and mode signatures. When the laser frequency is scanned, each of the photodiodes a and b detects an oscillating, near-sinusoidal signal. An XY-display on an oscilloscope screen yields a circle, where the momentary phase angle represents the laser frequency. The completed circumference of the circle corresponds to the range of the frequency scan. A full circle equals a frequency shift of one FSR of the interferometer. The radius additionally reveals information on the mode properties of the laser. A mode-hop free scan yields a smooth curve, whereas a mode-hop within the scan range is recognized by a sudden jump across the circle.

Options & Related Products

Options:

IR version with InGaAs photodetectors
Customized free spectral range of etalon
Twin iScan: Two measurement heads with a single control unit

Related Products:

	PID 110	LIR 110	PDD 110	FALC 110	DigiLock 110	LaseLock	Wavelength Meter	Iscan
Description / type	Analog PID	Analog Lock-In Regulator	Pound-Drever- Hall Signal Generation	Fast Analog 2-Channel PID	Versatile Digital Locking Solution	Suited for third party lasers	Wavelength meter with PID option	Low finesse etalons as reference with Lock-Electronics
Side-of-fringe	•			•	•	•	Not applicable
Top-of-fringe	With PDD	•	•	With PDD	•	•	Not applicable
Locking bandwidth*	kHz .. MHz	kHz range		≈ 45 MHz	≈ 10 MHz	1 MHz	< 100 Hz	kHz range
Modulation frequency		0.6 Hz .. 14 kHz	20 MHz **		17 Hz .. 25 MHz	33 Hz .. 1 MHz	Not applicable
Accuracy	Depends on reference (can be very good)						> 10 MHz	1 MHz (relative)
Signal analysis					•
Relock mechanisms	•	•			•	•		•
Computer control					•		•	•
High voltage output	•	•				•		•
High bandwidth output				•	•
Two channel version			•
SYS DC 110 module	•	•	•	•	•
Stand-alone						•	•	•
Catalog page	35	36	36	35	37	40	55	42
	low budget		high end & preferrable

*Estimated bandwidth depends on gain and PID settings **5- 40 MHz versions available

Tunable diode lasers: DL pro, DL 100 / pro design, DL DFB, DL 100
High power lasers: TA pro, DLX
Frequency-converted lasers: DL SHG pro, TA SHG pro, TA FHG pro
Laser packages for cw terahertz generation

Technical Drawings

Dimensions

iScan head: 80 x 80 x 114 mm³
Control rack: 134 x 300 x 445 mm³

Applications

Water absorption spectrum, recorded with a tunable diode laser with iScan frequency control. Left: 37 GHz scan across two absorption lines. Right: Frequency stepping. After the linear scan, the frequency is tuned in four discrete steps to the first resonance, an “off-line” value, the second resonance and the start of the scan ramp, respectively.

Linear 1200 GHz frequency scan of a DL DFB laser with iScan frequency control.

The iScan lends itself to

Frequency control of semiconductor lasers, fiber lasers, solid state lasers, dye lasers
Optimization of laser tuning parameters
Long-term aging control
LIDAR seeding
Phase-shifting interferomtery
Precision spectroscopy in frequency-domain terahertz applications

Application Notes