Nicolai Matuschek

Theory of double-chirped mirrors

A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated /spl ap/30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses.

Analytical design of double-chirped mirrors with custom-tailored dispersion characteristics

We present a theory for the analytical design of double-chirped mirrors with special dispersion characteristics. A simple analytical equation takes an arbitrarily desired group delay dispersion (that also includes possible higher order dispersion) as an input function and gives the chirp law as an output. The chirp law determines the local Bragg wavelengths in the mirror. It allows the calculation of the thicknesses of the high- and low-index layers if the double chirp of the layers in the front part of the mirror is taken into account. We use this method to design a highly dispersive double-chirped semiconductor Bragg mirror and a double-chirped TiO/sub 2/-SiO/sub 2/ mirror for higher order dispersion compensation in optical parametric oscillators operating in the visible spectral range. The design formulas are applicable to general chirped Bragg gratings and provide insight into the reasons why certain dispersion characteristics might be impossible to achieve.

Exact coupled-mode theories for multilayer interference coatings with arbitrary strong index modulations

The analysis of multilayered interference coatings by coupled-mode theory is considered to be an approximation useful for small index, gain, and/or loss modulations. In this paper, we show that an exact analysis of a multilayer coating with coupled-mode theory is possible by redefining the coupling and detuning coefficients. We derive the correct coefficients for the case of a Bragg mirror consisting of layers with arbitrary high and low refractive indexes. A detailed comparison with coupled-mode theories using the standard coupling and detuning coefficients is presented.

L-I characteristics of fiber Bragg grating stabilized 980-nm pump lasers

Light versus current (L-I) characteristics, of fiber Bragg grating stabilized 980-nm pump lasers, is experimentally studied and theoretically modeled. It is shown that a conventional design of such laser modules can result in sudden transitions between a coherence-collapse multimode emission, and a coherent single-mode state of operation.

Further development of high-power pump laser diodes

AlGaAs/InGaAs based high power pump laser diodes with wavelength of around 980 nm are key products within erbium doped fiber amplifiers (EDFA) for todays long haul and metro-communication networks, whereas InGaAsP/InP based laser diodes with 14xx nm emission wavelength are relevant for advanced, but not yet widely-used Raman amplifiers. Due to the changing industrial environment cost reduction becomes a crucial factor in the development of new, pump modules. Therefore, pump laser chips were aggressively optimized in terms of power conversion and thermal stability, which allows operation without active cooling at temperatures exceeding 70°C. In addition our submarine-reliable single mode technology was extended to high power multi-mode laser diodes. These light sources can be used in the field of optical amplifiers as well as for medical, printing and industrial applications. Improvements of pump laser diodes in terms of power conversion efficiency, fiber Bragg grating (FBG) locking performance of single mode devices, noise reduction and reliability will be presented.

980 nm single mode modules yielding 700 mW fiber coupled pump power

Fiber coupled light output power of highly reliable single mode laser diodes with an emission wavelength of around 980 nm has been increased by 40% as compared to former results. The devices reach in excess of 1.3 W ex-facet CW light output power with up to 60% power conversion efficiency and exhibit a vertical far-field of around 21/spl deg/. The maximum fiber coupled CW light output power amounts to more than 700 mW. The devices are designed to satisfy the stringent requirements of future cost efficient uncooled applications as well as the high power market of broadband multi channel networks.

External feedback optimization by means of polarization control in fiber Bragg grating stabilized 980-nm pump lasers

Effects of polarization change in the feedback provided by fiber Bragg gratings that stabilize 980-nm pump lasers are investigated. The effective feedback in the proper transverse-electric polarization is calculated from an easily measured parameter, describing the polarization at the fiber grating. This provides a useful experimental tool to investigate the effect of feedback loss due to polarization change that can occur in nonpolarization maintaining fibers. On the basis of our calculations, we give guidelines for optimum fiber layout to minimize polarization change.

Modeling and Simulation of Superluminescent Light-Emitting Diodes (SLEDs)

The availability of analytical models and numerical simulation tools is inevitable for the development and optimization of broadband high-power superluminescent light-emitting diodes (SLEDs) and its applications. In this paper, various theoretical aspects of SLEDs are discussed, which are important for the successful design of new devices with superior performance. We study the suppression of residual facet reflections as well as the importance of a careful vertical waveguide design. Furthermore, a simple analytical model for the L-I characteristics of SLEDs is developed that is based on a power law with an exponent that is dependent on the chip length. The theoretical model is verified by a comparison with experimental results of a broadband SLED operating in the wavelength region around 1300 nm. It is shown that the model can be also used to extract important simulation parameters from measured L-I characteristics. Finally, results are presented for an improved high-performance SLED structure in the same wavelength region with output powers of more than 50 mW and a 10-dB spectral bandwidth beyond 100 nm.

Highly efficient 980 nm single mode modules with over 0.5 Watt pump power

We present narrow beam divergence InGaAs/AlGaAs 980 nm ridge waveguide laser diodes with over 1 W CW-rollover power. Together with superior power conversion efficiency a maximum fiber-coupled light output power of over 0.5 W has been reached.

Improved brightness on broad-area single emitter (BASE) modules

In this communication we report on the approaches to increase the brightness of Bookhams latest generations of high power pump modules. Since the single-emitter laser diode is the essential building block in all module designs, the optimization of the device design towards higher wall-plug efficiency, higher brightness and better reliability is one focus of the ongoing development efforts at Bookham. By using an analytical simulation tool critical parameters for efficient emitter-fiber coupling as the beam divergence and coupling scheme could be identified.