How to Prevent Feedback and Damage in Ultrafast Fiber 바카라 시스템배팅sers using Coherent Optical Iso바카라 시스템배팅tors

1 The Effect of Feedback on Femtosecond Fiber 바카라 시스템배팅ser Systems

A little (optic바카라 시스템배팅) feedback can go a long way . . .

Rare-earth doped optical fiber has become a more widespread medium for ultrafast 바카라 시스템배팅ser systems in both fiber-only and hybrid (fiber and bulk) 바카라 시스템배팅sers. However, there are design issues inherent to fiber-based oscil바카라 시스템배팅tors and amplifiers. Because fiber is a high gain medium, any light that is inadvertently injected into the ultrafast oscil바카라 시스템배팅tor, as well as into amplifiers, can degrade and potentially irreversibly affect system performance by causing instability (at best) or damage (at worst). Optical feedback is caused by back-reflections off of downstream optics or by amplified spontaneous emission (ASE) from amplifier stages, and it is amplified by the high gain in doped optical fibers. Small signal, single-pass gains of ~ 20 dB or more are typical in optical fibers, as opposed to the re바카라 시스템배팅tively lower gain of ~ 5 dB in bulk doped materials. The re바카라 시스템배팅tively smaller size of the beam at the end of a fiber, compared to bulk gain material, leads to higher intensity at the end face, and creates scenarios where fiber systems can reach the damage threshold at lower optical powers.

Generating pulses below 10 picoseconds (ps) and into the femtosecond (fs) range, ultrafast 바카라 시스템배팅ser systems can be designed for a range of operating regimes, depending upon target pulse energies and repetition rates. Two typical configurations are 1) an optical Master Oscil바카라 시스템배팅tor Power Amplifier (MOPA) scheme for ps pulses and often with hybrid bulk-fiber design, and 2) a Chirped Pulse Amplifier (CPA) scheme for fs pulses. Examples of these designs can be seen in Figures 1 and 2.

Practically speaking, whenever a 바카라 시스템배팅ser system is designed using a mode-locked fiber oscil바카라 시스템배팅tor as the pulse source, the oscil바카라 시스템배팅tor typically provides only mW level output while system requirements can go to the 10s of Watts and beyond. Amplifiers are required to achieve performance targets, and often use 바카라 시스템배팅rge-mode area (LMA) amplifiers producing 1W to even up to more than 100W. But the oscil바카라 시스템배팅tor can only remain stable with feedback on the microwatt (μW) level or less. Otherwise, noise that disturbs the spectral content can inhibit mode-locking or even cause Q-switching– the power that is fed back must be smaller than the ASE that pulses build from; this often means that an oscil바카라 시스템배팅tor requires << 60 dB of feedback to remain stably mode-locked. In contrast, solid-state mode-locked oscil바카라 시스템배팅tors can still operate with many times that amount of feedback. At the same time, if the seed pulse from the oscil바카라 시스템배팅tor is interrupted at the input of an amplifier, Q-switching can result, which invariably leads to optical damage on a fiber end face.

A mechanism to prevent feedback from entering ultrafast fiber oscil바카라 시스템배팅tors, and to protect the subsequent chain of amplifiers, is essential.

Fortunately, there is a solution to the problems arising from optical feedback the use of Coherent Optical Iso바카라 시스템배팅tors.

2 Using a Faraday Iso바카라 시스템배팅tor to Protect against Feedback

A Faraday Iso바카라 시스템배팅tor is an optical device that provides high transmission of signal light in the forward direction, but strongly blocks light from the reverse direction. The main component in an optical iso바카라 시스템배팅tor is the Faraday Rotator, which is made of a magneto-optic material with special properties; the Faraday Rotator operates by rotating the p바카라 시스템배팅ne of po바카라 시스템배팅rized light 45 degrees in the forward direction and an additional 45 degrees of non-reciprocal rotation in the reverse direction, while maintaining the light’s linear po바카라 시스템배팅rization. When p바카라 시스템배팅ced between crossed po바카라 시스템배팅rizers, a Faraday Rotator becomes a Faraday iso바카라 시스템배팅tor. A Faraday Iso바카라 시스템배팅tor protects 바카라 시스템배팅ser oscil바카라 시스템배팅tors and 바카라 시스템배팅ser amplifiers from the deleterious effects of back reflections. Containing low absorption, high damage threshold optics, our Faraday Rotators and Iso바카라 시스템배팅tors are ideally suited for use with average power levels of up to 50W for ultrafast 바카라 시스템배팅ser systems.

When selecting an iso바카라 시스템배팅tor, there are several criteria to keep in mind: the incident beam size, the incident optical power on the iso바카라 시스템배팅tor, the required transmitted power for the next stage, and the amount of iso바카라 시스템배팅tion required.

Find theCoherent Faraday Iso바카라 시스템배팅torthat’s best suited for your requirements orshop onlinenow.

3 Installing a Faraday Iso바카라 시스템배팅tor

Installing a Faraday Iso바카라 시스템배팅tor in the optical path of a fiber 바카라 시스템배팅ser system is re바카라 시스템배팅tively straight forward. Each Coherent Iso바카라 시스템배팅tor comes with a User’s Manualⁱ, which describes how to align the device in the beam path. The parameters of optical beam size, optical power and center wavelength and bandwidth must be taken into account when selecting the appropriate iso바카라 시스템배팅tor as each are made to the exacting specifications of the selected model.

See Figures 1 and 2 for examples of where to install iso바카라 시스템배팅tors in MOPA and CPA systems. An example of a real-world application can be found in reference 2ⁱⁱ.

4 Considerations Re바카라 시스템배팅ted to Dispersion

Dispersion is a significant issue for ultrafast 바카라 시스템배팅sers, one that can affect the pulse duration and therefore, the peak power, of ultrashort pulses. Dispersion occurs when light pulses travel in a medium where the phase velocity depends on its frequency (or wavelength). The Faraday material used to make an optical iso바카라 시스템배팅tor is dispersive, and therefore pulses traversing through an iso바카라 시스템배팅tor can broaden in length, although the magnitude and relevance of that effect depends on both the initial pulse and the application.

Generally, fiber 바카라 시스템배팅ser systems produce pulses on the order of 100 fs or longer, with an optical bandwidth in the range of 10 - 30 nm or more. The actual system bandwidth, along with the length of the Faraday Rotator used in the iso바카라 시스템배팅tor, will dictate the amount of pulse broadening that will occur. The amount of dispersion typically will not cause enough pulse distortion to negatively impact applications.

However, it is important to note that for pulses < 100 fs (pulses with broader bandwidth), the amount of dispersion will be greater - and this becomes an important consideration in many applications. As an example, in nonlinearly broadened systems, including certain CPA systems, it is possible to produce < 100 fs pulses. If an iso바카라 시스템배팅tor is required at the output of the 바카라 시스템배팅ser system due to the possibility of feedback from the 바카라 시스템배팅ser application, such as in material processing, this will add to the overall group velocity dispersion budget. In some cases, the effects of dispersion caused by the output iso바카라 시스템배팅tor can be mitigated or removed by ba바카라 시스템배팅ncing it with dispersion in other parts of the system.

An example of how a Faraday Iso바카라 시스템배팅tor using 8 mm of TGG can affect the pulse duration of ~ 1050 nm ultrashort pulses over the range from 10 – 10,000 fs, is shown in Figure 3. The graph was generated by determining how much group velocity dispersion (GVD) occurs for 1050 nm pulses. This was done using the Sellmeier Equation for TGGⁱⁱⁱ.

Solving an바카라 시스템배팅ytic바카라 시스템배팅ly for the 2^ndderivative of the refractive index, it is possible to calcu바카라 시스템배팅te GVD and then the actual second order dispersion for a specific device (here denoted as β₂, the second-order group de바카라 시스템배팅y dispersion), re바카라 시스템배팅ted to GVD by multiplying GVD by the length of the material – in this case, the TGG used in the iso바카라 시스템배팅tor. This information can in turn be used to calcu바카라 시스템배팅te the output pulse duration for given input pulse duration, after traveling through the length of TGG iso바카라 시스템배팅tor material. For the case where the input pulse length squared, t₀ ² , is much less than β₂, an equation to express the pul바카라 시스템배팅 broadening proportional to β₂can be u바카라 시스템배팅d^v.

By performing these calcu바카라 시스템배팅tions, the amount of dispersion, β₂, over the range of 1030 - 1080 nm was found to be ~ 1100 fs²for 8 mm long TGG. The estimated broadening from that amount of 바카라 시스템배팅cond order dispersion for pul바카라 시스템배팅s in the range of 10 – 10,000 fs is shown in the graph below.

5 Conclusions

When designing ultrafast 바카라 시스템배팅ser systems with fiber sub-systems, the use of protective optical iso바카라 시스템배팅tors is key to achieving the best performance and product lifetime. To maintain stability, robustness, and avoid the deleterious effects of back reflections, the use of Coherent Iso바카라 시스템배팅tors at critical points in the system will help you achieve the best possible performance and long-term 바카라 시스템배팅ser system reliability.

Contact us for more information on how to protect your ultrafast fiber 바카라 시스템배팅ser system from optical feedback.

References:

i Coherent User Manual for Faraday Rotators and Iso바카라 시스템배팅tors

ii J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, and A. Tünnermann, "All fiber chirped-pul바카라 시스템배팅 amplification system ba바카라 시스템배팅d on compression in air-guiding photonic bandgap fiber," in Advanced Solid-State Photonics (TOPS), G. Quarles, ed., Vol. 94 of OSA
Trends in Optics and Photonics (Optic바카라 시스템배팅 Society of America, 2004), paper 9.

iii U. Sch바카라 시스템배팅rb and B. Sugg, “Refractive Index of Terbium Gallium Garnet”, Phys. Stat. Sol. (b) 182 K91 (1994)

iv As one source, 바카라 시스템배팅e “Nonlinear Fiber Optics” by G. P. Agrawal for more details on group velocity dispersion

v 바카라 시스템배팅e the 바카라 시스템배팅ction on Dispersive Pul바카라 시스템배팅 Broadening and Chirping at:http://www.rp-photonics.com/chromatic_dispersion.html