Selected Papers

High-power and Wide-band 1.3-µm Super Luminescent
Diode for Optical Coherence Tomography
Mitsuru Sugo† and Yasuo Shibata
Abstract
In this paper, we describe high-power (>50 mW), wideband (>50 nm) 1.3-µm super luminescent
diodes (SLDs). The SLDs have a tilted stripe and both facets have a window structure and antireflection
coating. This design reduces the spectrum ripple (<0.4 dB). An SLD module exhibited a high coupling
efficiency of over 70% and a high power of more than 30 mW in a single-mode fiber. These SLDs are
suitable for use in optical biological imaging.

1. Introduction
Super luminescent diodes (SLDs), which emit lowcoherent light, are used in optical measurement systems, such as fiber gyroscopes [1] and optical time
domain reflectometers (OTDRs) [2]. Recently, they
have become crucial elements for biophotonic imaging techniques such as optical coherence tomography(OCT)* [3]-[5]. OCT is already being put to practicaluse in retinal examination equipment, which uses a
0.83-µm SLD as a light source [3]. However, there arestill demands to extend the capabilities of OCT. For
instance, integrating OCT with endoscopes would
make it possible not only to observe the outer layersof internal organs but also to obtain information about
their inner structures [5], which would be helpful inthe early detection and treatment of cancer. These
applications require SLDs that emit light with awavelength of 1.3 µm [5]. In this region, water andhemoglobin (the constituents of living tissue thatabsorb the most light) both exhibit little light absorption and the light is less susceptible to scattering, soliving tissue exhibits the greatest transparency at thiswavelength. Consequently, the light can penetrate
deep into the body from the surface of the skin, allowing deep tomographic imaging.
These applications also require an SLD with higher
power and larger bandwidth than conventional ones

[6]. For in-vivo OCT imaging, the information-bearing signals tend to be weak and obscured by noise.
Accordingly, there has been a growing need for ahigh-output light source that can be used to obtainhighly precise data signals from living subjects. Thebandwidth of light emitted from the light source hasa large effect on the resolution of OCT images. Abroader bandwidth increases the resolution, resulting
in images with greater detail and allowing finer tissue
structures to be observed.
In this paper, we present 1.3-µm SLDs with a chip
output power above 50 mW and a large bandwidth of
more than 50 nm. A novel structure reduced spectrum
ripple to less than 0.4 dB even above 50 mW. An SLD
module exhibited a high coupling efficiency of morethan 70% and a high power of more than 30 mW in asingle-mode fiber. The cavity length dependence ofthe power and bandwidth are also discussed.
2. SLD structure
An InGaAsP multiple quantum well (MQW) active
layer with separate confinement heterostructure layers and a pn-buried heterostructure were grown bymetalorganic vapor phase epitaxy. To suppress spectrum ripple due to lasing in the Fabry-Perot mode
during high-power operation, we used a tilted stripe† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: sugo@aecl.ntt.co.jp* OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures. Only the coherence length of the light sources limits thedepth resolution of OCT.

Vol. 3 No. 8 Aug. 200521

Selected Papers(around 1 µm wide), gave both facets a window structure, and applied an antireflection coating to bothfacets. These techniques reduced the reflectivity ofthe facets. The structure of our SLD is schematicallyillustrated in Fig. 1.
The correlation between the root-mean-square(rms) maximum spectrum ripple and chip output
power for the novel SLDs is shown in Fig. 2, withdata for the old type of SLDs also shown for comparison. The old-type SLDs have a straight stripe, anantireflection coating on only the front facet, and awindow structure on only the rear facet. Spectrum
ripples for the old-type SLDs increase sharply withincreasing output power and result in lasing above
about 10 mW, which is undesirable. In our new SLDs,
on the other hand, spectrum ripple was suppressed tobelow 0.4 dB even at high output power above 50mW.

where Γ is the confinement factor, gm is the material
gain, and α i is the internal loss. The solid red line inFig. 3 is the fitting curve obtained from eq. (1). Equation (1) can explain the correlation between the output power and cavity length.
Next, let us see how this compares with an FP-LD
with both facets cleaved. The equation for the powerof an LD as a function of cavity length isP∝ Jop / [α i / ln(1/R)+1/L],(2)

where Jop is the current per unit length and R is thefacet reflectivity. It is well known that an LD outputs
more power than an SLD. However, as the cavity
becomes longer, the rate of the increase in the power
gradually drops (dotted blue line), while that for theSLD (solid red line) rises. Eventually, the powers
become comparable. This dispels our preconceptionthat an SLD cannot emit high-power light like an LD.

3. SLD output power
4. Bandwidth and module performance of SLD
The suppression of spectrum ripple enabled us toobtain higher power using a longer cavity length. Figure 3 shows the correlation between the measured
output power and cavity length for our SLDs (closedcircles). For comparison, data for a Fabry-Perot laser
diode (FP-LD) are also shown (open circles). The
measurement condition was a constant current perunit length of 100 mA/mm. For an SLD, a simplified
model can express output power (P) as a function ofcavity length (L) as [7]P∝ exp [(Γgm-α i)L] – 1,

Window structure

Front facet

Rear facet

Tilted stripe

Antireflection coating

5(1)Antireflection coating

Maximum spectrum ripplerms (dB)Window structure

The correlation between the 3-dB bandwidth (fullwidth at half maximum) of the spectrum and cavity
length is shown in Fig. 4. The open circles are measured values and the line is a guide for the eye. Current density was again constant at 100 mA/mm. The
bandwidth decreased gradually with increasing cavity length. The 3-dB bandwidth value of 120 nm
obtained by extrapolating the cavity length to zero

4Old-type SLDs
32Novel SLDs
100102030

405060

Output power (mW)

Fig. 1. Schematic structure of our SLD.

22Fig. 2. Correlation between maximum spectrum rippleand chip output power.

NTT Technical Review

Selected Papers

corresponds to the bandwidth of spontaneous emission from the MQW active layer. The bandwidth wasalso a function of the injection current density. Figure 5 shows (a) typical light-current characteristics
obtained in the current range up to 600 mA and (b)the spectrum at a current per unit length of 233
mA/mm (Po = 50 mW). In this case, the cavity length
was 1.5 mm. Normally, there is a trade-off between

bandwidth and optical output and this has prevented
the development of SLD light sources that combine ahigh optical output with a large bandwidth. However,
our SLD achieved both high output power of over 50mW and large bandwidth of more than 50 nm simultaneously.
We have also developed an SLD module using ahermetically sealed 14-pin butterfly package shown

50160
140
Jop = 100 mA/mm

401203-dB bandwidth (nm)Power (mW)

Jop = 100 mA/mm

30FP-LD
20100
8060
4010SLD
000.5

1201.50200.5

Cavity length (mm)11.5

2Cavity length (mm)Fig. 3. Correlation between output power and cavity
length.

Fig. 4. Correlation between 3-dB bandwidth and cavity
length.

80–10Jop = 233 mA/mm
Po = 50 mW

70–20L = 1.5 mm

52.7 nm–3050Intensity (dB)

Output power (mW)

60

40
30–40

–50
20–60
10
0

0

100

200

300

400

500

600–701.2

1.25

1.3Current (mA)

Wavelength (µm)(a)(b)

1.35

1.4

Fig. 5. (a) Typical light-current characteristics and (b) spectrum.

Vol. 3 No. 8 Aug. 2005

23

Selected Papersin Fig. 6. Figure 7 shows typical measured characteristics of light output from a single-mode fiber
attached to the module versus current supplied to themodule and the measured voltage-current characteristics of the module. The coupling efficiency exceeded 70% and the output power was more than 30 mW.
This high coupling efficiency helped to reduce themodule’s power consumption. At 30 mW, the operating current was as low as 300 mA and the operating
voltage was as low as 1.7 V.

high chip output power (>50 mW) and large bandwidth (>50 nm). The novel structure ensured lowspectrum ripple (<0.4 dB) even above 50 mW. An
SLD module exhibited a high coupling efficiency ofover 70% and a high power of more than 30 mW in asingle-mode fiber. These SLDs are suitable for use in
optical biological imaging systems.
References[1]

5. Conclusions[2]

In summary, we demonstrated 1.3-µm SLDs with
[3]

[4]

[5]

[6]

[7]

K. Bohm, P. Marten, K. Peterman, E. Weidel, and R. Ulrich, “Lowdrift fiber gyro using a super luminescent diode,” Electron. Lett., Vol.
17, pp. 352-353, 1981.
K. Takada, I. Yokohama, K. Chida, and J. Noda, “New measurement
system for fault location in optical waveguide devices based on aninterferometric technique,” Appl. Opt., Vol. 26, pp.1603-1606, 1987.
D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson,W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G.
Fujimoto, “Optical Coherence Tomography,” Science, Vol. 254, pp.
1178-1181, 1991.
M. Akiba, K. P. Chan, and N. Tanno, “Full-field optical coherence
tomography by two-dimensional heterodyne detection with a pair ofCCD cameras,” Opt. Lett., Vol. 28, pp. 816-818, 2003.
A. M. Rollins, M. V. Sivak, Jr., S. Radhakrishnan, J. H. Hass, D.
Huang, K. D. Cooper, and J. A. Izatt, “Emerging clinical applicationsof optical coherence tomography,” Optics & Photonics News, Vol.
13, pp. 36-41, Apr. 2002.
H. Okamoto, M. Wada, Y. Sakai, T. Hirono, Y. Kawaguchi, Y.
Kondo, K. Kishi, and Y. Itaya, “A narrow beam 1.3-µm-super luminescent diode integrated with a spot-size converter and a new typerear absorbing region,” J. Lightwave Tech., Vol. 16, pp. 1881-1887,
1998.
J. Wang, H. Olsen, and K. E. Stubkjaer, “Recombination, gain andbandwidth characteristics of 1.3-µm semiconductor laser amplifiers,”
J. Lightwave Tech., Vol. LT-5, pp. 184-189, 1987.

Fig. 6. SLD module using a hermetically sealed 14-pin
butterfly package.

50540430

320210100100

200

300

4000Voltage (V)Fiber output power (mW)

Mitsuru SugoSenior Research Engineer, Supervisor, ProjectLeader, High Temperature Operation Laser
Development Project, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inphysics from Hokkaido University, Sapporo,Hokkaido in 1982 and 1984, respectively. He
received the Ph.D. degree in electronics engineering from the University of Tokyo, Tokyo in
1993. In 1984, he joined NTT Laboratories,
where he has been studying III-V compound
semiconductor optical devices. He is a memberof the Japan Society of Applied Physics (JSAP)and the Institute of Electronics, Information andCommunication Engineers (IEICE) of Japan.

Yasuo Shibata
Senior Research Engineer, Photonic Device
Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Keio University, Kanagawa in 1985 and 1987, respectively. In 1987, hejoined NTT Optoelectronics Laboratories, Kanagawa, where he has been engaged in research on
optical switches and optical filters. He is currently with NTT Photonics Laboratories, engaged inresearch on SOA and wavelength-conversion
devices. He is a member of JSAP and IEICE.

Current (mA)

Fig. 7. Typical light-output power and voltage/current
characteristics of SLD module.

24NTT Technical Review