Poul Varming

Fiber laser optical frequency standard at 1.54 μm

A 32 mW fiber laser is stabilized to the (13)C(2)H(2) P(16) (ν1 + ν3) transition at 1542 nm using saturated absorption. The short-term shot-noise limited fractional frequency instability is 5.0 × 10(-13)(τ/s)-½ for averaging times τ up to about 100 s. The relative lock-point repeatability over 2½ month is 4.3 × 10(-13) corresponding to 83 Hz. The simple setup includes a 21 cm long gas cell, but it does not require an enhancement cavity or external modulators. The spectroscopic lineshape is analyzed with respect to optical power and acetylene pressure. Narrow linewidths of 300 kHz FWHM are observed with a signal to noise ratio of 35 dB in a 9 Hz bandwidth.

Polarization control method for ultraviolet writing of advanced Bragg gratings

We present a flexible and simple method for UV writing of Bragg gratings with advanced apodization profiles including discrete phase shifts. The method is based on a p phase shift between the refractive-index modulation profiles induced by s and p polarization of UV light. By changing the ratio of UV intensity in the two polarizations we are able to control the modulation strength and to induce phase shifts while keeping a constant effective refractive index throughout the Bragg grating. We demonstrate strong UV-written Bragg gratings with Gaussian or sinc apodizations with spectral shapes, in good agreement with theoretical predictions.

Single-frequency thulium-doped silica DFB fiber laser at 1735 nm

We have developed a single-frequency thulium doped silica fiber laser with a Distributed FeedBack (DFB) cavity operating at a wavelength of 1735 nm. The laser cavity is 5 cm long formed by a UV-written Bragg grating with a phase shift and is pumped by a Ti:Sapphire laser at 790 nm. The laser operates in a single-polarization mode and is tunable over a few nanometers. To the best of our knowledge, this represents the first short cavity, single frequency fiber laser using thulium as the amplifying medium. The lasing wavelength is among the lowest demonstrated in a thulium-doped fiber laser and it falls in an attractive near-to-mid infrared wavelength region only offered by few sources. Single-frequency DFB fiber lasers are compact and stable optical sources, which offer low-noise coherent output with ultra-narrow-linewidth. Typical applications for DFB fiber lasers are as sources for coherent sensing, spectroscopy and several high-end applications. Using optical fiber doped with erbium and/or ytterbium these sources provide emission within the wavelength bands of 980 - 1200 nm and 1525 - 1620 nm. A thulium doped DFB laser opens a new broad wavelength range from 1.7 μm - 2.0 μm, depending on co-dopants. This wavelength range is especially interesting for use in gas sensors, frequency mixing and as a source for eye-safe LIDAR applications.

Applications of single frequency fiber lasers

In this paper we address fiber Bragg Grating (FBG) based fiber lasers, which consist of a rare earth single mode fiber with a FBG imposed in the fiber. The rare earth doped optical fiber is typically pumped with a semiconductor pump. These laser sources further exhibits extremely narrow spectral linewidth in the order of kHz and very low phase noise in the low frequency area making the single frequency fiber laser very attractive for many coherent applications such as interferometry, mid infrared generation, absolute reference sources, coherent communication, LIDAR and, due to the capability to be used in harsh environments, for down the well applications measuring pressure and temperature.

Design and fabrication of Bragg grating based DFB fiber lasers operating above 1610 nm

Distributed feedback fiber lasers operating at wavelengths above the L-band an presented for the first time. Furthermore, a numerical analysis that predicts the optimum design of these lasers has been developed.

91-km attenuation-free transmission with low noise accumulation by use of distributed erbium-doped fiber

Transparency of a 91-km distributed erbium-doped fiber is achieved with 0.46 mW/km of pump power at a signal power of -12 d Bm. The accumulation of amplif ier noise is measured to be smaller than the minimum noise accumulation that can be achieved in a 91-km link with two lumped amplifiers separated by 45 km.

Comparison of photosensitivity in germanium doped silica fibers using 244 nm and 266 nm continuous-wave lasers

Diode pumped continuous-wave UV lasers offer an interesting alternative to frequency doubled argon-ion lasers. We report the first photosensitivity comparison using these lasers on deuterium loaded standard telecommunication fibers and unloaded experimental fibers.

DFB fiber laser as source for optical communication systems

The results demonstrate that DFB fiber lasers are an attractive alternative as sources in telecommunication systems. The lasers show excellent long-term stability with very high signal to noise ratio and a reasonable output power, combined with exceptional temperature stability and inherent fiber compatibility.

New method for fabrication of advanced UV written Bragg gratings

A new simple method for fabricating advanced Bragg gratings with arbitrary profiles is presented. Apodization and phase shifts are controlled by the polarization of the UV beam and does not require movement of the phase mask or fiber during exposure.

Dynamic response analysis of DFB fibre lasers

We present a model for relative intensity noise (RIN) in DFB fibre lasers which predicts measured characteristics accurately. Calculation results implies that the RIN decreases rapidly with stronger Bragg grating and higher pump power. We propose here a simplified model based on three spatially independent rate equations to describe the dynamic response of erbium doped DFB fibre lasers on pump power fluctuations, using coupled-mode theory to calculate the steady-state hole-burning of the erbium ion inversion.