Superluminescent Diode Devices

superluminescent diode

Superluminescent Diodes (SLDs) (or Superluminescent Light Emitting Diodes (SLEDs)) are optoelectronic semiconductor devices that emit broadband optical radiation based on superluminescence. In terms of construction, they are similar to laser diodes, containing an electrically driven p–n junction and an optical waveguide, but SLDs intentionally lack optical feedback, so that no laser action can occur. Optical feedback, which could lead to the formation of resonator modes and thus to pronounced structures in the optical spectrum and/or to spectral narrowing, is suppressed by means of tilting the facets relative to the waveguide, and can be suppressed further with anti-reflection coatings. Essentially, an SLD is a semiconductor optical amplifier with no input signal, where weak spontaneous emission into the waveguide mode is followed by strong laser amplification (therefore, this is called “amplified spontaneous emission (ASE)”).

SLDs are applied in situations where a smooth and broadband optical spectrum (i.e. low temporal coherence), combined with high spatial coherence and relatively high intensity, is required.

Center Wavelength(nm)
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Typical Ripple(dB) Typical Current
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Package Type Part Number
Center Wavelength(nm) Typical 3dB Bandwidth(nm) Typical Output Power(mW) Typical Ripple(dB) Typical Current Package Type Part Number
750 10 3 0.1 120 BUT or DIL IPSDD0701
750 14 10 0.1 120 TO 8, 9 or 56 Ex-Window IPSDT0701
770 13 8 0.1 180 BUT or DIL IPSDD0705
770 20 5 0.1 140 BUT or DIL IPSDD0706
780 12 3 0.1 150 BUT or DIL IPSDD0702
780 12 10 0.1 180 BUT or DIL IPSDD0707
780 40 5 0.1 200 BUT or DIL IPSDD0708
800 10 15 0.1 200 BUT or DIL IPSDD0809
800 40 5 0.1 200 BUT or DIL IPSDD0810
820 15 0.3 0.1 120 BUT or DIL IPSDD0801
820 15 5 0.1 120 TO 8, 9 or 56 Ex-Window IPSDT0801
820 15 8 0.1 140 TO 8, 9 or 56 Ex-Window IPSDT0802
820 25 2.5 0.1 140 BUT or DIL IPSDD0802
820 25 8 0.1 140 TO 8, 9 or 56 Ex-Window IPSDT0803
820 40 5 0.1 180 BUT or DIL IPSDD0803
830 30 15 0.2 200 BUT or DIL IPSDD0820
830 32 45 0.1 250 TO 8, 9 or 56 Ex-Window IPSDT0804
830 40 7 0.1 200 BUT or DIL IPSDD0812
830 40 10 0.1 150 TO 8, 9 or 56 Ex-Window IPSDT0805
830 50 5 0.1 150 BUT or DIL IPSDD0813
840 35 5 0.1 160 BUT or DIL IPSDD0804
840 45 8 0.1 200 BUT or DIL IPSDD0807
840 45 11 0.1 250 BUT or DIL IPSDD0808
840 50 8 0.1 200 BUT or DIL IPSDD0823
850 50 8 0.1 200 BUT or DIL IPSDD0815
870 50 6 0.1 180 BUT or DIL IPSDD0816
880 45 6 0.1 200 BUT or DIL IPSDD0805
880 40 2 0.1 180 BUT or DIL IPSDD0806
880 45 8 0.1 180 BUT or DIL IPSDD0819
880 55 5 0.1 180 BUT or DIL IPSDD0817
900 15 20 0.2 200 BUT or DIL IPSDD0902
900 15 35 0.2 200 TO 8 or 9 Ex-Window IPSDT0901
900 30 10 0.1 200 TO 8 or 9 Ex-Window IPSDT0902
900 45 7 0.1 200 BUT or DIL IPSDD0903
920 30 3 0.1 150 BUT or DIL IPSDD0901
920 55 8 0.1 200 BUT or DIL IPSDD0904
920 90 5 0.1 200 BUT or DIL IPSDD0905
980 25 5 0.1 250 BUT or DIL IPSDD0906
1020 100 10 0.15 250 BUT or DIL IPSDD1001
1020 60 7 0.1 150 BUT or DIL IPSDD1005
1020 110 8 0.1 300 BUT or DIL IPSDD1006
1040 55 30 0.2 400 BUT or DIL IPSDD1007
1040 70 10 0.1 250 BUT or DIL IPSDD1002
1050 45 35 0.2 400 BUT or DIL IPSDD1008
1050 55 15 0.1 300 BUT or DIL IPSDD1009
1050 55 30 0.1 400 BUT or DIL IPSDD1003
1070 60 5 0.1 500 BUT or DIL IPSDD1010
1070 60 10 0.15 400 BUT or DIL IPSDD1004
1280 55 10 0.5 350 BUT or DIL IPSDD1201
1280 70 5 0.15 300 BUT or DIL IPSDD1202
1280 95 10 0.5 500 BUT or DIL IPSDD1203
1310 40 1.5 0.1 120 TO 8, 9 or 56 Ex-Window IPSDT1301
1310 40 0.5 0.1 120 TO 56 pigtail Ex-Fiber IPSDT1303
1310 40 5 0.1 150 TO 8, 9 or 56 Ex-Window IPSDT1302
1310 40 35 1 400 BUT or DIL IPSDD1305
1310 45 1 0.1 120 BUT or DIL IPSDD1301
1310 45 20 1 350 BUT or DIL IPSDD1302
1310 45 25 1 350 BUT or DIL IPSDD1309
1310 50 15 0.2 150 TO 8, 9 or 56 Ex-Window IPSDT1310
1310 55 7 0.5 300 BUT or DIL IPSDD1303
1310 55 20 1 450 BUT or DIL IPSDD1304
1310 55 25 1 350 BUT or DIL IPSDD1311
1310 70 18 1 500 BUT or DIL IPSDD1306
1310 65 15 1 250 BUT or DIL IPSDD1312
1310 80 15 1 450 BUT or DIL IPSDD1307
1310 90 10 1 350 BUT or DIL IPSDD1313
1310 100 3 0.1 180 BUT or DIL IPSDD1308
1410 50 10 1 300 BUT or DIL IPSDD1401
1410 60 15 1 450 BUT or DIL IPSDD1402
1410 70 10 1 550 BUT or DIL IPSDD1403
1490 50 5 0.5 200 BUT or DIL IPSDD1404
1490 65 18 1 500 BUT or DIL IPSDD1405
1520 50 15 0.15 400 BUT or DIL IPSDD1505
1520 75 10 1 350 BUT or DIL IPSDD1506
1550 40 0.2 0.15 120 TO 56 pigtail Ex-Fiber IPSDT1501
1550 55 0.5 0.1 120 BUT or DIL IPSDD1501
1550 55 5 0.2 200 BUT or DIL IPSDD1502
1550 60 3 0.2 300 BUT or DIL IPSDD1503
1550 50 3 0.2 150 TO 8, 9 or 56 Ex-Window IPSDT1502
1550 60 10 1 300 BUT or DIL IPSDD1504
1550 65 12 0.15 300 BUT or DIL IPSDD1507
1550 65 20 0.4 450 BUT or DIL IPSDD1508
1550 90 8 1 300 BUT or DIL IPSDD1509
1580 60 5 0.2 300 BUT or DIL IPSDD1510
1580 75 5 0.4 300 BUT or DIL IPSDD1511
1610 55 2 0.1 250 BUT or DIL IPSDD1601
1610 55 5 0.5 250 BUT or DIL IPSDD1602
1640 40 5 0.5 400 BUT or DIL IPSDD1603
1640 50 3 0.5 200 BUT or DIL IPSDD1604

 

Applications of SLDs :

SLDs have been used for many different applications. The major application fields are

(1) Optical Coherence Tomography
(2) White light interferometry
(3) Fiber-optic link testing
(4) WDM PON systems
(5) Fiber-optic sensors
(6) Fiber-optic gyroscopes

The actual applications in each field are summarized in Table 1. More details are described in the following sections.


Table 1 Applications of SLDs

Fields
Applications
Wavelengths
Optical Coherence Tomography (OCT)
  • Cornea and retina diagnostics
  • Cardiovascular imaging
  • Biomedical purposes or biology research
  800 nm band

1050 nm band

1310 nm band

 

White light interferometry
  • Measuring the chromatic dispersion of optical components
  • Measuring the distances (surface profile) by using Michelson interferometer with a CCD camera as detector
  • Detecting the reflections within a photonic integrated circuit
  800 nm band

1310 nm band

1550 nm band

Fiber-optic link testing
  • Measuring the chromatic dispersion or polarization mode dispersion of fiber-optic links
1310 nm band

1550 nm band

WDM PON systems
  • Broad-band seed light source
1550 nm band
Fiber-optic sensors
  • Measuring the temperature, strain or pressure in buildings, oil pipelines or oil fields (structural health; premises securitization)
1550 nm band
Fiber-optic gyroscopes
  •  Avionic navigation
  800 nm band

1550 nm band

 

Optical Coherence Tomography (OCT) Applications :

Optical coherence tomography (OCT) is an optical signal acquisition and processing method using an interferometric technique to capture micrometer-resolution, three-dimensional images from within optical scattering media such as biological tissue. SLDs have been used as a broad-band spectrum light source. Very wide-spectrum emitting over a ~100nm wavelength range have achieved sub-micrometer resolution. Optical coherence tomography typically employs near infrared light. The use of relatively long wavelength light allows it to penetrate into the scattering medium.

Commercially available optical coherence tomography systems are employed in diverse applications, including art conservation and diagnostic medicine, notably in ophthalmology where it can be used to obtain detailed images from within the retina. Recently it has also begun to be used in interventional cardiology to help diagnose coronary artery disease.

There are two types of OCT: (1) Time-domain OCT, and (2) Frequency-domain OCT.

(a) Time-domain OCT :

Basic configuration of time–domain OCT system is shown in Fig. 1 [1]. Light in an OCT system is broken into two arms—a sample arm (containing the item of interest) and a reference arm (usually a mirror). The combination of reflected light from the sample arm and reference light from the reference arm gives rise to an interference pattern, but only if light from both arms have travelled the “same” optical distance (“same” meaning a difference of less than a coherence length). By scanning the mirror in the reference arm, a reflectivity profile of the sample can be obtained.

The axial and lateral resolutions of OCT are decoupled from one another; the former being an equivalent to the coherence length of the light source and the latter being a function of the optics. The coherence length of a source and hence the axial resolution of OCT is defined as,

where Δλ is the 3dB bandwidth of the SLD spectrum, and λ0 is the central wavelength.

 (b) Frequency-domain OCT :

In frequency domain OCT, the broadband interference is acquired with spectrally separated detectors, either by encoding the optical frequency in time with a spectrally scanning source or with a dispersive detector, like a grating and a linear detector array. One example of the frequency-domain OCT configuration with using a spectrally scanning source, is shown in Fig. 2 [2]. Due to the Fourier relation (Wiener-Khintchine theorem between the auto correlation and the spectral power density), the depth scan can be immediately calculated by a Fourier-transform from the acquired spectra, without movement of the reference arm. This feature improves imaging speed dramatically, while the reduced losses during a single scan improve the signal to noise proportional to the number of detection elements. The parallel detection at multiple wavelength ranges limits the scanning range, while the full spectral bandwidth sets the axial resolution.

Basic configuration of time-domain OCT
Fig. 1 Basic configuration of time-domain OCT [1]
 

 

Basic configuration of frequency-domain OCT using Swept source or tunable laser
Fig. 2 Basic configuration of frequency-domain OCT using Swept source or tunable laser [2]

White Light Interferometry :

White light interferometry is to capture intensity data at a series of positions along the vertical axis where the surface is located by using the shape of the white-light interferogram, the localized phase of the interferogram, or a combination of both shape and phase.

An example of the operation principle is illustrated in Fig. 3. The light of a broadband light source (SLD) with short coherence length is split into two beams: an object beam and a reference beam. The object beam reflects from the object (sample), and the reference beam reflects off of a reference mirror. The two reflected beams are captured and recombined at the beam splitter. The superimposed beams are imaged by a CCD camera for processing. If the optical path for an object point in the measurement arm is the same as the optical path in the refrence arm, there is constructive interference, which results in a high intensity in the camera pixel of the respective object point, for all wavelengths in the spectrum of the SLD. For object points having a different optical path, the interference is destructive, which results in a much lower intensity. In this way, the topolographical structure of the sample is converted to light intensity difference, and, therefore, to the CCD output signals, which are compiled and analyzed.

One example of white light interferometry application is to measure the surface roughness on semiconductor wafers [3].

 

White light interferometry basic configuration
Fig. 3 White light interferometry basic configuration.

 

Fiber-Optic Link Testing :

SLDs are used in the diagnostics of optical fiber communication networks in the 1310nm and 1550nm bands. The chromatic dispersion of an optical medium is the phenomenon that the phase velocity and group velocity of light propagating in a transparent medium depend on the optical frequency. Dispersion has an important impact on the propagation of optical pulses, because a pulse always has a finite spectral width (bandwidth), so that dispersion can cause its frequency components to propagate with different velocities. Normal dispersion, for example, leads to a lower group velocity of higher-frequency components, and thus to a positive chirp, whereas anomalous dispersion creates negative chirps. The frequency dependence of the group velocity also has an effect on the pulse duration. If the pulse is initially unchirped, dispersion in a medium will always increase its duration (dispersive pulse broadening).

In optical fibers, there is usually some slight difference in the propagation characteristics of light waves with different polarization states. This is called polarization mode dispersion (PMD) [3]. A differential group delay can occur even for fibers that according to the design should have a rotational symmetry and thus exhibit no birefringence. This effect can result from random imperfections or bending of the fibers, or from other kinds of mechanical stress, and is also affected by temperature changes. Mainly due to the influence of bending, the PMD of a cabled fiber can be completely different from that of the same fiber on a spool. Modern fiber cables as used in fiber-optic links have been optimized for low PMD, but the handling of such cables can still have some influence. PMD can have adverse effects on optical data transmission in fiber-optic links over long distances at very high data rates, because portions of the transmitted signals in different polarization modes will arrive at slightly different times. Effectively, this can cause some level of pulse broadening, leading to inter-symbol interference, and thus a degradation of the received signal, leading to an increased bit error rate.

The chromatic dispersion and polarization mode dispersion (PMD) can be measured by using the large bandwidth, high power spectral density, and low ripple characteristics of SLDs.

WDM PON Systems :

The Wavelength Division Multiplexing (WDM) Passive Optical Networks (PON) has been used and developed as one of the approaches for Fiber To The Home (FTTH) network systems [5][6]. As a low-cost laser source at the Optical Network Unit (ONU) in such WDM PON systems, the Fabry Perot (FP) laser diode (LD) wavelength is locked to a selected wavelength channel of the broad-band Amplified Spontaneous Emission (ASE) source.[6]. Fig. 4 shows an architecture for upstream transmission employing wavelength-locked FP LDs [6]. A broad-band ASE source (such as SLD) with an optical circulator is located at the central office. The broad-band ASE is transmitted to the remote node where an Arrayed Waveguide Grating (AWG) slices the ASE spectrally. The spectrally sliced ASE is injected into the FP LD located at the ONU.

 

WDM PON System Upstream Configuration
Fig. 4 WDM PON System Upstream Configuration

 

Fiber-Optic Sensors (FOS) :

 

(a) Advantages of FOS

 

(b) Types of FOS

There are two types of fiber optic sensors:

 

(c) Intrinsic Sensors

The optical characteristics of an optical fiber is sensitive to the strain, temperature, and pressure, which modulates the intensity, phase, polarization, wavelength, or transit time of light in the fiber. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over very large distances.

Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is best suited for this environment as it functions at temperatures too high for semiconductor sensors (distributed temperature sensing).

Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy using fiber Bragg gratings. This is particularly useful when acquiring information from small complex structures. Brillouin scattering effects can be used to detect strain and temperature over larger distances (20–30 kilometers). The fiber sensor is especially useful for harsh environments.

Example: Bragg grating sensors for strain and temperature

A schematic diagram of the fiber Bragg grating sensor is shown in Fig. 5.

The Bragg wavelength can be expressed as,

 

where n is the mode index of the fiber, and Λ is the grating period. Equation (1) gives,

 

where ΔλB,  Δn and ΔΛ express a small change of λB, n, and Λ, respectively.

 

Fiber Bragg grating sensor configuration for temperature and strain measurement
Fig. 5 Fiber Bragg grating sensor configuration for temperature and strain measurement

 

A fiber-optic AC/DC voltage sensor in the middle and high voltage range (100–2000V) can be created by inducing measurable amounts of Kerr nonlinearity in single mode optical fiber by exposing a calculated length of fiber to the external electric field. The measurement technique is based on polarimetric detection and high accuracy is achieved in a hostile industrial environment.

 

High frequency (5MHz–1GHz) electromagnetic fields can be detected by induced nonlinear effects in fiber with a suitable structure. The fiber used is designed such that the Faraday and Kerr effects cause considerable phase change in the presence of the external field. With appropriate sensor design, this type of fiber can be used to measure different electrical and magnetic quantities and different internal parameters of fiber material.

 

Electrical power can be measured in a fiber by using a structured bulk fiber ampere sensor coupled with proper signal processing in a polarimetric detection scheme. Experiments have been carried out in support of the technique.

 

Hydrophone systems with more than one hundred sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries’ navies. Both bottom-mounted hydrophone arrays and towed streamer systems are in use.

 

Fiber optic microphone and fiber-optic based headphone find application in areas with strong electrical or magnetic fields, such as communication amongst the team of people working on a patient inside a magnetic resonance imaging (MRI) machine during MRI-guided surgery.

 

(d) Extrinsic Sensors

Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. Extrinsic fiber optic sensors provide excellent protection of measurement signals against noise corruption:

 

Fiber-Optic Gyroscopes :

The interferometric fiber optic gyroscope (IFOG) uses an optical interferometer for the very high-resolution readout of the Sagnac phase shift induced between two counter-propagating waves in an optical closed path when the plane of propagation undergoes angular rotation. The basic scheme is illustrated in Fig. 6. It is a passive interferometer where the fiber optic coupler is employed to split the radiation from the light source into two counter-propagating waves, clockwise (CW) and counterclockwise (CCW), in the fiber coil and to recombine the waves, after propagation, on a photodetector PD. The phase difference is thus cumulated over a long fiber coil for obtaining high responsivity with a compact device. For ideal fibers and components, the output photo-generated current I has the following expression:

where φS is the so-called Sagnac phase shift, and

where σ is the photodetector responsivity and P is the power coupled into the input fiber.

The Sagnac phase shift φS is given by (see Fig.7),

where

 

Basic scheme of the fiber optic gyroscope
Fig. 6 Basic scheme of the fiber optic gyroscope (FOG)