Optical Fiber Measurements

Various laboratory measurements are routinely performed on telecommunication fibers to test their performance as components of fiber-optics communication systems. Some of these measurements are listed below.

	Attenuation is the loss of optical power as a result of absorption, scattering, bending, and other loss mechanisms as the light travels through the fiber. The total attenuation is a function of the wavelength λ of the light. The total attenuation A between two arbitrary points X and Y on the fiber is A(dB) = 10 log₁₀ (P_x/P_y). P_x is the power output at point X. P_y is the power output at point Y. Point X is assumed to be closer to the optical source than point Y. The attenuation coefficient or attenuation rate α is given by α(dB/km) = A/L. Here L is the distance between points X and Y. The cutback method is often used for measuring the total attenuation of an optical fiber. The cutback method involves comparing the optical power transmitted through a long piece of fiber to the power transmitted through a very short piece of the fiber. The cutback method requires that a test fiber of known length L be cut back to a length of approximate 2 m. It requires access to both ends of the fiber. The cutback method begins by measuring the output power P_y of the test fiber of known length L. Without disturbing the input conditions, the test fiber is cut back to a length of approximate 2 m. The output power P_x of the short test fiber is then measured and the fiber attenuation A and the attenuation coefficient a are calculated. Different launch conditions can lead to different results. For multimode fiber, the distribution of power among the modes of the fiber must be controlled. This is accomplished by controlling the launch spot size, i.e. the area of the fiber face illuminated by the light beam, and the angular distribution of the light beam. When the launch spot size is smaller than the area of the fiber face and the numerical aperture NA of the input radiation is smaller than the NA of the fiber, the fiber is said to be underfilled. Most of the optical power is concentrated in the center of the fiber and mainly low-order modes are excited. underfilled launch conditions When the launch spot size is larger than the area of the fiber face and the numerical aperture NA of the input radiation is larger than the NA of the fiber, the fiber is said to be overfilled. Light that falls outside the fiber core and light incident at angles greater than the angle of acceptance of the fiber core is lost. Overfilling the fiber excites both low-order and high-order modes. overfilled launch conditions Launch conditions affect the results of multimode fiber attenuation measurements. If too much power is launched into high-order modes, the high-order-mode power loss will dominate the attenuation results. Generally, fiber attenuation measurements are performed using underfilled launch conditions.
	The cutoff wavelength of a single mode fiber is the wavelength above which the fiber propagates only the fundamental mode. We need V = k_f a NA = 2π a NA / λ₀ < 2.405. The cutoff wavelength of a single mode fiber is a function of the fiber's radius of curvature. Measuring the cutoff wavelength involves comparing the transmitted power from a test fiber with that of a reference fiber as a function of wavelengths. The test fiber is loosely supported in a single-turn with a constant radius of 140 mm. The transmitted signal power P_s(λ) is recorded while scanning the wavelength in increments of 10 nm or less over the expected cutoff wavelength. The launch and detection conditions are not changed while scanning over the range of wavelengths. For the reference power measurement the launch and detection conditions are not changed, but the fiber is bent to a radius of 30 mm or less to suppress the second-order mode at all the scanned wavelengths. The transmitted signal power P_r(λ) is recorded while scanning over the same wavelength range as before. The attenuation at each wavelength is calculated. P_s(λ) = 10^-As(λ)/10, P_r(λ) = 10^-Ar(λ)/10, P_s(λ)/P_r(λ) = 10^{-(As(λ)-Ar(λ))/10} = 10^Ad(λ)/10. The attenuation A_d at each wavelength is calculated. A_d(λ)(dB) = 10 log₁₀ (P_s(λ)/P_r(λ)). The longest wavelength at which A_d(λ)/km is equal to 0.1 dB is the fiber cutoff wavelength.
	The modal bandwidth of a multimode optical fiber can be measured by measuring the power transfer function H(f) of the fiber at the band frequency (f). Signals of varying frequencies (f) are launched into the test fiber and the power exiting the fiber at the launched fundamental frequency is measured. This optical output power is denoted as P_out(f). The test fiber is then cut back or replaced with a short length of fiber of the same type. Signals of the same frequency are launched into the cut-back fiber and the power exiting the cut-back fiber at the launched fundamental frequency is measured. The optical power exiting the cutback or replacement fiber is denoted as P_in(f). The magnitude of the optical fiber frequency response is defined as H(f) = 10 log₁₀(P_out(f)/P_in(f)), or, if the launch conditions for the two experiments are not exactly the same, H(f) = 10 log₁₀(P_out(f)P_in(0)/P_in(f)P_out(0)). The fiber bandwidth is defined as the frequency at which the magnitude of the fiber frequency response has decreased to one-half its zero-frequency value and H(f) = -3. This frequency is called the -3 decibel (dB) optical power frequency (f_3dB) and referred to as the fiber bandwidth. Bandwidth is normally given in units of megahertz-kilometers (MHz-km). Converting to a unit length assists in the analysis and comparison of optical fiber performance.
	Chromatic dispersion occurs because the index of refraction is a function of wavelength and different wavelengths of light travel through the fiber at different speeds. The chromatic dispersion of multimode graded-index and single mode fibers is obtained by measuring the time it takes pulses of light with different wavelengths to travel through a long piece of fiber. These measurements are made using multi-wavelengths sources such as wavelength-selectable lasers or multiple sources of different wavelengths.
	To make fiber geometry measurements, the input end of the fiber is overfilled and mode filtered. The output end of the fiber is viewed with a video camera. The image from the video camera is sent to a computer for digital analysis. The computer analyzes the image to identify the edges of the core and cladding. The cladding diameter is defined as the average diameter of the cladding. The core diameter is defined as the average diameter of the core. Cladding noncircularity, or ellipticity, is the difference between the smallest radius of the cladding and the largest radius of the cladding divided by the average cladding radius. Core noncircularity is the difference between the smallest core radius and the largest core radius divided by the average core radius. Core noncircularity is measured on multimode fibers only. The core-cladding concentricity error for multimode fibers is the distance between the core and cladding centers divided by the core diameter. The core-cladding concentricity error for single mode fibers is defined as the distance between the core and cladding centers.

	The core diameter is measured by measuring the power distribution in the near-field region close to the fiber-end face, when the distance between the fiber-end face and detector is in the micrometers range. The core diameter (D) is defined as the diameter at which the intensity is 2.5 percent of the maximum intensity.
	The numerical aperture (NA) is a measurement of the ability of an optical fiber to capture light. It is determined by measuring the far-field power distribution in the region far from the fiber-end face. The emitted power per unit area is recorded as a function of the angle θ some distance away from the fiber-end face. The distance between the fiber-end face and detector in the far-field region is in the centimeters (cm) range for multimode fibers and millimeters (mm) range for single mode fibers. The fiber NA is defined by the 5 percent or 0.05 intensity level, This 0.05 intensity level intersects the normalized curve at scan angles θ_A and θ_B. The fiber NA is defined as NA = sin((θ_A- θ_B)/2).
	The mode field diameter (MFD) of a single mode fiber is related to the spot size of the fundamental mode. This spot has a mode field radius r₀. The mode field diameter is equal to 2r₀. Single mode fibers with large mode field diameters are more sensitive to fiber bending. Single mode fibers with small mode field diameters show higher coupling losses at connections. The mode field diameter of a single mode fiber can be measured by measuring the output far-field power distribution of the single mode fiber using a set of apertures of various sizes.
	Insertion loss is composed of the connection coupling loss and additional losses in the fiber following a connection. In multimode fiber, fiber joints can increase fiber attenuation following the joint by disturbing the mode power distribution. In single mode fibers, fiber joints can cause the second-order mode to propagate in the fiber following the joint. To measure insertion loss, power measurements are made on an optical fiber before the joint is inserted and after the joint is inserted. Initial power measurements at the detector (P₀) and at the source monitoring equipment (P_M0) are taken before inserting the interconnecting device into the test setup. The test fiber is then cut and the device is inserted. After insertion the power at the detector (P₁) and at the source monitoring equipment (P_M1) is measured again. The insertion loss is calculated as Insertion loss (dB)= 10 log₁₀((P₁/P₀)(P_M0/P_M1))
	Reflection occurs at optical fiber connections. The reflectance R is the fraction of the incident intensity that is reflected back into the source fiber at the point of the connection. The return loss is defined as Return loss = -10 Log R. The reflectance R is measured using an optical source connected to one input of a 2 X 2 fiber optic coupler. Light is launched into the component under test through the fiber optic coupler. The light reflected from the component under test is transmitted back through the fiber optic coupler to a detector connected to the other input port.