R&D Information

the nano-filter shown in Fig. 5. The first exposure anddevelopment formed the framework of the filter, andthe second formed an array of small holes on the sidewalls of the framework. The smallest holes have adiameter of about 30 nm.
Future prospects
Plans call for the development of methods of applying this technique to nanofabrication of various mate-rials, such as semiconductors. In addition, investigations will be carried out to determine how this technique can be used to make new types of highly functional nanoelectronic devices.
For further information, please contact
NTT Science and Core Technology Laboratory
GroupAtsugi-shi, 243-0198 Japan
E-mail: st-josen@tamail.rdc.ntt.co.jp

Compact Lasers Generate Light of any Desired
Wavelength Filling the Wavelength Gaps ofSemiconductor Lasers—Suitable for Applications
in Biotechnology, Medicine, and Environmental
Measurement

NTT has developed a technique for making solidstate lasers that can be designed to emit at a specificdesired wavelength within a certain range, which had
been impossible with semiconductor lasers. This newtechnology is being promoted through the “Comprehensive Producer Function*1”, a program that wasstarted in July 2003 to promote the commercialization of prominent research achievements. Allianceswith other corporations should lead to commercial
products on the market in summer 2004. This newtechnology provides a high output, a longer operating
life, and easy maintenance in a palm-sized device.
The lasers are also less expensive than conventionallasers. We expect great demand in the fields wherelarge gas lasers are currently used and also in biomedical fields such as DNA or cell function analysis
and environmental measurement of NOx or SOx*1 Comprehensive producer function: Special producers appointed
for commercialization directly promote the prominent research of
NTT Laboratories to put products on the market in cooperation
with NTT Group companies and other outside companies.

82(oxides of nitrogen and sulfur).
Background
Lasers are categorized by their emission sources.
For example, there are gas lasers (e.g., the argonlaser), solid-state lasers (e.g., the Nd:YAG laser), andsemiconductor lasers. Semiconductor lasers havebecome more common because they are small, light,and inexpensive. They are used mainly for the visible
and near-infrared regions for recording media including CDs and DVDs as well as in the telecommunication field (Fig. 1).
However, there are wavelength gaps for whichsemiconductor lasers are not available: the 0.5–0.6µm region (green, yellowish-green, yellow, orange)
and the 2–5-µm region (mid-infrared). For thesewavelengths, other emission sources are used andthey are more expensive and consume more power
than semiconductor lasers. NTT has developed aseries of compact lasers that cover these regions.
Each laser can be designed to generate a specific
NTT Technical Review

R&D Information

Commercializedlaser diodes
Communication applications

Pump LDs for Nd:YAG lasers

Non-communication applications
Pump LDsfor fiber amplifiers(980 nm, 1480 nm)

barchip

Output (mW)

1000

100

DVD

Blu-rayCD

LDs forcommunications

10

Data communications
10.4

0.6

0.8

1.0

1.3

1.55

2Wavelength (µm)

The wavelength gaps not covered by laser diodes.

Fig. 1. Commercially available laser diodes.

desired wavelength. Our goal was to create a “custom-made compact laser”.
Technological points

length is a function of the two input wavelengths, soby choosing available lasers with appropriate input
wavelengths, we can obtain a specific desired output
wavelength. This technique is not new, but wave-The essence of the technology is to combine theoutputs from two semiconductor lasers using a highly efficient optical non-linear crystal*2 for optical
wavelength conversion (Fig. 2). The output wave-*2 Optical non-linear crystal: A crystal whose refractive or absorption index depends on the intensity of the incident light. It is used
for wavelength conversion including optical modulation and second harmonic generation (SHG).

A laser at a desired wavelength is provided by combining λ1 and λ2.
LD-1
λ1

LD-2
λ21λ3

=1
λ1

±1
λ2λ3
Optical non-linear crystal

Fig. 2. Technological principle.

Vol. 2 No. 4 Apr. 2004

83

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length mixing is usually very inefficient. NTT Photonics Laboratories has been researching and developing wavelength converting technology for opticalcommunication using LiNbO3 (lithium niobate)*3 as
the optical nonlinear crystal. As a result, we deviseda new method of producing crystals that exhibit highconversion efficiency and reliability. A conventional
LiNbO3 waveguide exhibits photorefractive damage
when it is injected with high-power pump light. We
fabricated a ridge waveguide using the direct-bonding technique. This waveguide shows high resistanceto photorefractive damage. We succeeded in obtaining a converted light intensity ten times higher thanusual by combining an optical non-linear crystal
designed for out-of-band communication wavelengths and high-output telecommunication-use
semiconductor lasers. This technology can produce alaser that emits light with any desired wavelength in
the region from 500 nm to 5 µm (Figs. 3 and 4).
Technical features
The laser’s features are described below.
• Output power is 10 mW (30 mW is also available)• Smaller size and lower operating power than aconventional gas laser

OrangeYellowish
greenGreenFig. 3. Compact lasers at specific wavelengths.

• Easier to maintain than a gas laser because of its
long operating life
• No need for an external modulator because directmodulation is possible• Continuous operation at room temperature is possible even in the mid-infrared region (>2 µm).
Applications*3 LiNbO3 (lithium niobate) exhibits the highest electro-opticaleffect of all practical materials. It is widely used for high-speed
modulators for optical communication.

The expected application areas are shown below
and summalized in Table 1.

1210Power (mW)864

20586588590592594

Wavelength (nm)Fig. 4. Example of LD emission spectrum.

84

NTT Technical Review

R&D Information

Table 1. Product lineup.
Product group

Visible laser

Laser for gas
analysis(single longitudemode)

Output power, etc.

Main application fields

0.5–0.6 µm(green, yellowish
green, yellow,orange)

3 mW (CW)(>10 mW class underdevelopment)

Flow cytometers used inbio-fields, such as cell andDNA analysis, source of
excitation light for laser
microscopes, etc.

0.65–0.9 µm(near-infrared)

1 mW (CW)

2–5 µm(Mid-infrared)

approx. 1 mW
(under development)
Room temperature CW
operation

Wavelength

Light sources for sensors ofenvironmental and variousorganic gases, etc.

CW: continuous wave• Flow cytometers or confocal laser microscopes,
which are used for applications in biotechnology
such as cell or DNA analysis• Light sources for sensors of environmental gas
such as carbon dioxide (CO2), nitrogen oxides(NOx), sulfur oxides (SOx), and various organic
gases• Replacement for sodium D-line lamps used toanalyze the components of medical products,foods, and chemicals
We also expect our lasers to meet demands for newor replacement products in various wavelength
regions. For this purpose, we are considering
alliances with Kyoto Electronics Manufacturing Co.,

Vol. 2 No. 4 Apr. 2004Ltd. and SC Bioscience Corporation.
Prospects
The technology is expected to be made commercially available by NTT Electronics Co. and marketed in summer 2004, taking full advantage of the“Comprehensive Producer Function”.
For further information, please contact
NTT Science and Core Technology Laboratory
GroupAtsugi-shi, 243-0198 Japan
E-mail: st-josen@tamail.rdc.ntt.co.jp

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