Difference Frequency Comb (DFC)

Inherently CEP-stable modular frequency combs

TOPTICA’s frequency comb product line is a modular system that supports a broad variety of applications. Three basic versions of the DFC are available: DFC CORE, DFC CORE+ and DFC SEED. All models use TOPTICA proprietary CERO-technology to achieve an unprecedented low-noise performance.

The DFC CORE and its high performance version DFC CORE+ come with a digital oscilloscope for beat monitoring and a GPS disciplined RF reference included. They both provide 4 or optionally 8 phase-stable outputs at 1560 nm.

Several extension modules are available that can convert the DFC CORE / DFC CORE+ outputs to any wavelength between 420 nm and 2200 nm. The extension modules can be upgraded at any time after purchase of the DFC CORE and are interchangeable between outputs.

In addition, beam combiner (DFC BC) and beat detector (DFC MD) units are available to provide RF beats between the DFC comb lines and cw lasers. The RF output signal of the DFC MD can be counted to determine the frequency of the cw laser. It also enables phase or frequency stabilization of the cw laser to the DFC using e.g. TOPTICA’s locking modules mFALC or DigiLock. In the DFC CORE+ version, the RF output signal can also be used to stabilize the DFC to the cw laser which serves as optical reference.

Such a DFC system can be combined with any of TOPTICA’s tunable diode lasers to achieve a complete, frequency referenced laser system including wavelength meter and counter all from one source.

Key Features TOPTICA DFC Frequency Combs

Patented CERO-technology („zero-f_CEO”) with intrinsic CEP stability
RMS phase stability < 35 mrad
Frequency stability < 8·10^-14 @ 1s (measured with RF reference), or same as reference
< 0.04 % RMS power fluctuations
One free parameter f_rep with up to 3 control elements (oscillator temperature, piezo, pump current) and > 400 kHz bandwidth

DFC - Integrated Components

	DFC CORE	DFC CORE+	DFC SEED
Oscillator incl. piezo f_rep	✔	✔	✔
Fiber-optic amplifier and supercontinuum	✔	✔	✔
DFG unit with CERO technology	✔	✔	✔
Fiber optic pre-amplifier 1560 nm	✔	✔	✔
Lock electronics for f_rep	✔	✔	✔
Control computer	✔	✔	✔
4 or optionally 8 outputs at 1560 nm	✔	✔
Fiber optic amplifier and nonlinear wavelength conversion			✔
2 CEP-stable outputs: 1560 nm and 1030 nm or 1950 nm + 3 optional outputs at 1560 nm			✔
Digital oscilloscope for beat monitoring	✔	✔
RF reference: Oven-controlled quartz, GPS disciplined	✔	✔
f_rep stabilization option to optical reference (does not include optical reference)		✔

DFC - Characteristics

One free parameter f_rep− 3 actuators

Locking to an optical reference is achieved through feedback onto the pump current of the oscillator. The use of an EOM is not necessary. This keeps the oscillator design simple and avoids piezo-electric resonances usually present in EOMs.

Actuator	Oscillator temperature	Piezo	Pump current
Locking bandwidth	1 Hz	70 kHz	> 400 kHz

Spectral Interferometry

The performance of the CERO-technology is best characterized by means of spectral interferometry. The dedicated setup consists of an f-2f interferometer with optical spectrum analyzer which records optical fringes at the output. It measures the absolute phase stability of the f_CEO cancelation with respect to an independent reference. The figure shows a spectrogram of the interference fringes recorded over a time of 20 seconds. The high interference contrast is stable over the full time span and no offset phase drift is observed. The measurement shows a RMS phase stability of 8 mrad over 20 s, which is limited by the stability of the f-2f interferometer. The real phase stability of the DFC is expected to be below this value.