ABSTRACT
Examples of data sheets for two industry standard passive optical bers are shown in Table 5.1. One of these bers (1060-XP) is an industry standard ber widely used in low power (mostly telecommunication) applications. The other is a ber almost exclusively used for high-power ber lasers, LMA-GDF-20/400 (standing for large-mode-area, germanium-doped ber) and available from multiple ber suppliers (e.g., References 1 and 2). From this table we can identify some of the major differences between bers optimized for high-power ber lasers and those associated with more conventional telecom bers. First one can see that some of the specications are not actually measured on both sets of bers. This reects partly the different standards that have been adopted in the telecom and ber laser industries and also the difculty in measuring some values, particularly in the case of the LMA bers. The 1060-XP ber is primarily used around the wavelength range 980-1060 nm, and carrying low-power single-mode light at those wavelengths. Indeed the most common application for the 1060-XP ber is to pigtail to single-mode components such as laser diodes and couplers operating at 980 nm or 1060 nm. However, we can see that the operating wavelength range for the 1060-XP ber is stated to be 980-1600 nm, spanning the three common telecommunications wavelengths at 980 nm, 1300 nm, and 1550 nm. The ber is therefore specied to be single mode at 1060 nm by dening a cutoff wavelength specied at <920 nm, guaranteeing that the ber does not have more than one mode across the entire speci-ed operating wavelength range from 980 to 1600 nm. By comparison, the LMA-GDF-20/400 ber does not specify a cutoff wavelength and includes an operating wavelength regime, which is arguably rather unclear in this case since it does not refer to the single-mode regime of the ber. Rather, the LMA-GDF-20/400 ber species a core numerical aperture (NA) and core diameter d, which may be used to calculate a normalized frequency parameter (V value) at the desired operating wavelength λ, using equation 5.1, which can then be used to calculate the cutoff wavelength (a V value of 2.4 corresponds to the cutoff wavelength):
=
pi
λV d NA (5.1)
Unlike the telecom ber 1060-XP, which is designed to operate in the single mode regime over a given wavelength range, the LMA-GDF-20/400 ber is often used in a few-moded regime making the operating wavelength range for the LMA-GDF-20/400 ber rather redundant. For the ber parameters in Table 5.1, we estimate the V value for the LMA-GDF-20/400 to be 3.84 at 1064 nm, 2.63 at 1550 nm, and 1.99 at 2050 nm, as shown in Table 5.2 along with the number of modes the core supports at each of these operating wavelengths. Unlike the telecom industry, the adoption of these few-moded bers in ber laser applications has proved very useful in making higher power ber lasers, allowing higher powers to be both generated and transmitted because of the large core size and mode eld area (MFA). Since these bers are generally used in fairly short lengths, the mode mixing is low and resulting beam quality in the ber can be very good, as we will see throughout this book.
