In-fibre lasers: compact, efficient, full of potential

Southampton Photonics' new CEO David Parker says the company will soon evolve from "building the engines to building the car". The new "cars" are likely to include in-fibre lasers that offer great potential for optical communications.

By Matthew Peach

Southampton Photonics (SPI) has evolved from the Optoelectronics Research Centre of Southampton University, UK, over the past two years. The company focuses on three key technologies: doped and active fibres for EDFAs; speciality fibre structures; and a fibre grating technology platform.

In the company's mission statement, SPI says it intends to "master the chemistry of glass and the physics of light... we aim to be the leading supplier of in-fibre systems."

But how do these systems work and what benefits do they offer?

At a time when SPI is poised to grow from being an OEM supplier to becoming a fully-fledged solution provider, Lightwave Europe interviewed Dr Louise Hickey, SPI's senior engineer leading DFB optical developments (pictured right, Fig. 1).

Matthew Peach: What is an in-fibre laser?

Louise Hickey: An in-fibre laser is where both the laser gain medium and the feedback mechanism is contained in a continuous fibre. For example, a distributed feedback (DFB) fibre laser designed for telecoms applications contains a core doped with optically active ions and a photosensitive material for writing a Bragg grating. The Bragg grating provides feedback and the active ions in the core absorb a pump wavelength, emitting light at the lasing wavelength.

The use of the fibre geometry allows efficient, low-loss routing of a pump wavelength to the gain region. The pump mode can be matched to the circular symmetry and distribution of the gain medium, giving a highly efficient transfer of energy.

With a pump wavelength outside the Bragg reflector band, the pump is absorbed along the fibre, with no interaction with the grating. Resulting in spontaneous and stimulated emission at the Bragg wavelength of the grating, the emission is reflected within the cavity. The output is compatible with the fibre geometry and is seamlessly transferred to a transmission fibre using a standard splice.

MP: How do the IF-laser's wavelengths and powers compare with other sources?

LH: For a DFB fibre laser, only a single longitudinal mode can operate in the laser cavity, with the wavelength defined by the grating design. For a typical Er-Yb doped laser, the linewidth is extremely narrow at around 30kHz, compared to 1MHz of a conventional semiconductor transmitter.

The use of a standard glass host and long grating (compared to semiconductor lasers) ensures a stable environment for the optical power transfer between the pump and laser mode. This allows very narrow linewidths and excellent noise performance to be achieved; typically, the linewidth is less than 100kHz, the RIN is less than 150dB/Hz at above 50MHz and the signal-to-noise ratio is >65dB.

A range of output powers can be provided for differing applications. A standard output power of 10mW is readily achievable over the full C-band (1528-1565nm) for 100mW of pump power. As the efficiency and threshold are dependent on the grating design, specific needs can be addressed on a custom basis. For example, efficiencies of 40% are targeted, with output powers greater than 50mW.

MP: Why are IF lasers appropriate for telecoms applications?

LH: The wavelength stability offered by the Bragg grating technology means high precision, athermal packaging of lasers is a reality for metro applications. The elimination of thermo-electric coolers (TECs) reduces the electrical complexity and overall power consumption of transmitter board. Wavelength lockers are not required, nor is any on-site wavelength tuning by installation engineers. The overall impact is a reduction in power consumption and a reduction in network complexity.

The use of optical pumping offers greater architectural freedom, with the ability to separate the transmitter source card from the control electronics, such that a passive source card can be located remotely from the electronic systems. Based on a proven erbium-ytterbium (Er/Yb) glass, the laser offers a high signal-noise ratio and low RIN. This enables the laser source to be used for high data rate and high channel density transmission schemes. The high quality of the source will enable longer transmission distances between regeneration sites.

MP: How does the IF laser work?

LH: Our DFB fibre laser is based on the absorption of 980nm light and emission due to an optical transition of erbium. Using the ytterbium allows efficient absorption of the 980nm pump light, whilst the erbium emission is entirely compatible with the telecoms transmission wavelengths.

The energy transfer between the ions and the optical power conversion within the Bragg grating are shown (left, Fig. 2).

The 980nm pump light is delivered to the Yb/Er-doped region and efficiently absorbed by the Yb ion. At the pump wavelength, there is no interaction with the Bragg grating, so absorption is governed by material properties.

The absorbed power is transferred from the Yb to a nearby Er ion. The Er undergoes a non-radiative decay to an upper laser level. This is the same excited state that is used in the standard Er-doped amplifier and so exhibits the same spectral properties.

The lasing action starts with spontaneous emission from the upper laser level. Spontaneous emission captured by the fibre geometry interacts with the Bragg grating, such that a specific wavelength is reflected by the grating structure. With the presence of a phase shift in the grating, a resonant cavity is formed, which enables laser emission as the level of stimulated emission increases and the threshold conditions are met.

Therefore, the basic components of the DFB fibre laser are the core material, the formation of the grating and the delivery of pump power to the active region.

The use of the Er/Yb system allows an efficient absorption of the pump power, so the total fibre laser length is only several centimetres long. The output wavelength is determined by the Bragg grating design and the emission properties of the Er dopant. This combination ensures the laser wavelengths are entirely compatible with a telecoms product.

MP: Typical IF laser output is 10mW; how could the power be increased?

LH: The performance of the laser is controlled by both the grating design and the composition of the gain medium. Both may be optimised for particular performance spec, so customers requiring higher output powers can be satisfied, although the market potential will drive the different product paths.

Higher output powers in the 20-40mW range will enable more complex transmitter topologies and modulation formats and/or may obviate the need for booster amplifiers. The higher powers are necessary for the future implementation of high-spectral-efficiency (Bits/s/Hz) networks. The noise performance of the DFB fibre laser, coupled with higher operating powers, would present an excellent source for advanced network configurations.

MP: SPI first showed its commercialised

IF-lasers at OFC 2001, with a 16-channel unit; what has happened since?

LH: The 16-channel, 50GHz-channel-spacing array provides an excellent demonstration of DFB fibre laser technology. Over the past year, the manufacturing process has been refined, leading to systems tests, prototype supply and customer interaction. The development path has addressed wavelength coverage, high-efficiency operation, packaging and lifetime testing. Cost-effective module and array solutions with differing pump schemes are showing promise at delivering a radical economic solution to wavelength provisioning.

Fibre Bragg gratings are made by writing a fringe pattern onto the core of a photo-sensitive fibre. Depending on the nature of the pattern, specific wavelengths will be rejected, thus enabling a filtering function. The FBG writing process involves projecting UV light on to the core of the fibre, a task which traditionally requires a phase mask to be made in accordance with the particular specification required.

SPI has developed and implemented a different manufacturing technique by which the fringe pattern is written directly onto the core of the fibre. The characteristics of the grating are determined entirely by computerised control of the UV exposure during the fabrication process. The process recipe for a certain product is simply captured in a data file: an approach which completely removes the costly step of mask fabrication. SPI's no-mask approach facilitates a rapid development cycle, enabling closer customer involvement, and offers a low-cost route to developing new products. The process has significant advantages for volume manufacturing since the same product can be produced simultaneously on multiple manufacturing stations. "Direct writing" minimises manual labour by processing a batch of similar FBGs using state-of-the-art, bespoke equipment.

SPI's process revolves around algorithms, developed over a number of years, starting with work at the University of Southampton's Optoelectronic Research Centre. SPI's design techniques permit accurate realisation of complex filtering characteristics. There is no intermediate step between the design process and grating fabrication: the modelling tool and the fabrication equipment both use the same software and file format to describe the grating. The devices made with the SPI process are not subject to errors introduced during mask fabrication that cause problems of cross-talk and time delay ripples. This allows production of high-quality DWDM filters and dispersion compensators with negligible system penalty.