Optical plug system for optical measuring device

An optical plug connector for an optical measuring device between an internal light waveguide optical fiber and an external light waveguide optical fiber maintains a distance d between the ends of the fibers which is greater than the coherence length of the light to be transmitted through the coupling. In this manner, in spite of the presence of the air gap, interference phenomena are avoided.

FIELD OF THE INVENTION 
My present invention relates to an optical measuring assembly having a plug 
connection for joining an external light waveguide, e.g. an optical fiber, 
with an internal light waveguide (e.g. an optical fiber) and, more 
particularly, to an optical plug connection for optical measuring devices 
having internal light waveguides of this type. 
BACKGROUND OF THE INVENTION 
It is known to provide an optical plug connection for optical measuring 
devices for the transmission of light signals to or the transmission of 
light signals from the optical measuring device utilizing light 
waveguides, i.e. optical fibers. A plug connector can be provided on the 
housing of the measuring device, in particular for joining an internal 
light waveguide with an external light waveguide, i.e. for providing the 
optical coupling between them. Conventional measuring devices utilizing 
optical connectors of this type operate exclusively on the butt-contact 
principle. The light conducting cores of the two light waveguides to be 
interconnected lie precisely aligned and as tightly as possible against 
one another. 
Indeed, most plug connections of this type provide means for pressing the 
end face of one of the light waveguides against the end face of the other 
light waveguide to be coupled to the first. 
If the system is to include an optical transmitter with a large coherence 
length, for example a laser diode, and is to use glass fibers as the light 
waveguides, the conventional approach has been to ensure contact between 
the optical fiber cores to be coupled for two reasons: 
a) If an air gap would exist between the two fiber ends, a resonator would 
be formed because of the double glass-air reflection (glass-air 
interface/air-glass interface) which exists on both sides of the air gap. 
Even the smallest changes in the length of the air gap, for example, by 
temperature effects or changes in the laser wavelength or variations in 
the mode distribution in the case of multimode fibers (e.g. resulting from 
fiber bending) give rise to a change in the characteristics of the 
resonator and thus a variation in the transmission loss which can amount 
to several tenths of a dB. Such comparatively high loss fluctuations can 
be intolerable with measuring devices utilizing optical signals and, for 
this reason, in conventional systems every effort is made to avoid an air 
gap in the plug connector. 
b) Modern semiconductor laser diodes are highly sensitive to reflections. 
An air gap in the plug connector which can result in about 4% reflection 
at each glass/air interface cannot be tolerated for most applications of 
the system. On this ground as well, prior measuring devices required 
plug-type connectors in which the two fibers were pressed together at 
least in their respective core regions to ensure a reliable contact 
between them. 
The contacting fiber ends, however, have the disadvantage that even the 
smallest size dust or dirt particle in the contact region will suffice to 
cause the formation of a detrimental air gap and to so increase the 
transmission loss as to interfere with proper operation of the measuring 
device. In extreme cases, the fiber ends can be damaged. 
For conventional plug connectors, therefore, the coupling of the fibers 
together must be carried out in a dirt-free manner so that before the 
connection of one light waveguide with another, the ends of the waveguides 
to contact one another must be carefully cleaned. This is a time-consuming 
process. With an optical measuring device, plug connections are made and 
disconnected repeatedly and an optical fiber may have to be connected to a 
number of devices. The cleaning operations involved and the danger of 
damage to the fiber ends is thereby greatly increased. 
To avoid damage to the fiber ends in regions in which the fibers are to be 
coupled together in a plug, it is possible to provide between the fiber 
ends, a fluid light-conducting medium, for example, a so-called 
index-matching oil. However, the use of the oil has long-term effects 
which introduce undefinable inaccuracies and limit the reliability of the 
system. 
In the technical manual Feinwerktechnik & Messtechnik 96 (1988) 4, P. 151 
to 154, a plug connection has been described which provides a lens 
arrangement between the ends to be connected by the plug connector. A lens 
arrangement of this type reduces the effect of contamination in the region 
of an end face of a plug upon the damping by comparison with direct 
connection plugs. However this assembly is complex to manufacture and thus 
correspondingly expensive since the ball-type lenses used must be 
fabricated with high precision. 
Some background as to the problems of earlier connectors can be derived 
from German patent documents DE 35 06 844 and DE 31 48 562, as well as a 
publication in the names of Naumann/Schroder "Bauelemente der Optik", Carl 
Hanser Verlag Munchen Wien 1983, pp. 18, 19, 218, 219. 
These publications disclose various optical connecting systems and 
theoretical considerations of the transmission in the region of the plug 
connectors. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the present invention to provide 
an optical plug connector for optical measuring units which avoids the 
possibility of damage to the fiber ends and affords reproducible loss 
values for the connector. 
Another object of the invention is to provide an improved optical measuring 
assembly with a plug connector between an external light waveguide and an 
internal light waveguide whereby drawbacks of earlier systems are avoided. 
Still another object of my invention is to eliminate the specific problems 
outlined above with respect to systems having direct contact between fiber 
ends, systems requiring lenses to be interposed between them and systems 
using index-watching oil or other difficult-to-handle media to optically 
couple the fibers. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the invention, in an optical measuring 
assembly comprising: 
an optical measuring device for use with light having a coherence length 
l.sub.c and having an internal light waveguide having a connection end, 
and a plug connector into which the internal light waveguide extends at 
the connection end; 
an external light waveguide having a connection end extending into the plug 
connector and optically communicating therein with the connection end of 
the internal light waveguide for the transmission of the light 
therebetween; and 
means for maintaining a distance d between the connection ends which is 
greater than the coherence length l.sub.c. 
When the light is generated by a light-emitting diode, the distance d is 
greater than the coherence length l.sub.c =.lambda..sub.0.sup.2 
/.DELTA..lambda., where .lambda..sub.0 is the central maxiumum wavelength 
and .DELTA..lambda. is the spectral halfwidth of the light generated by 
the light-emitting diode. 
In a laser system, the optical measuring assembly can comprise: 
an optical measuring device having a laser light source producing coherent 
light, an internal light waveguide formed as a multimode optical fiber 
having a connection end, and a plug connector into which the internal 
light waveguide extends at the connection end; 
an external light waveguide having a connection end extending into the plug 
connector and optically communicating therein with the connection end of 
the internal light waveguide for the transmission of the light 
therebetween; and 
means for maintaining a predetermined distance between the connection ends. 
The invention can also include a method of reducing interference in an 
optical measuring assembly wherein an optical measuring device has a laser 
light source producing coherent light, an internal light waveguide formed 
as a multimode optical fiber having a connection end, and a plug connector 
into which the internal light waveguide extends at the connection end and 
an external light waveguide having a connection end is inserted into the 
plug connector and optically communicates therein with the connection end 
of the internal light waveguide for the transmission of the light 
therebetween, the method comprising the step of grinding back at least one 
of the ends for maintaining a predetermined distance between the 
connection ends within the plug connector, the plug connector being 
constructed to cause the ends to abut absent the grinding back. 
By providing the ends of the light waveguides which are coupled together in 
the plug connector at the distance d which is greater than the coherence 
length of the transmitted light, I am able to ensure that no interference 
can occur. 
The reflections at the glass/air interfaces, do indeed provide a loss but 
only a defined loss which is independent of small distance changes as long 
as the distance between the fiber ends is greater than the coherence 
length. 
The air gap between the fiber ends is thus chosen to be so large that plug 
tolerances and other effects on the air gap do not reduce the distance d 
to a value less than the coherence length in operation and use of the 
system. The plug connector of the invention has the advantage that one 
need not operate with inconvenient media such as immersion oil and 
nevertheless can eliminate the danger of damage to the fiber ends. 
The coherence length is defined by the equation l.sub.c 
=.lambda..sub.0.sup.2 /.DELTA..lambda.. 
From this equation, the following typical values are obtained for 
.lambda..sub.0 =1300 nm. 
l.sub.c .apprxeq.20 .mu.m for LED 
l.sub.c .apprxeq.1 m for Laser 
In the case of a light-emitting diode (LED), the air gap in the above case 
must be greater than or equal to 20 .mu.m to assure that no interference 
can occur as a result of the presence of the gap. 
A similar advantage can be obtained also for lasers by providing the 
predetermined distance between the connection ends wherein the internal 
light waveguide is a multimode fiber with high dispersion, for example, a 
step index fiber. In this case, a progation delay difference between the 
individual modes is obtainable. When this transit time difference is 
greater than the coherence time of the laser at the transmission gap 
between the fibers, no interference can occur. For a laser, as an optical 
transmitter, the coherence time amounts to: 
EQU t.sub.c &lt;1 ns. 
If one uses stepped index fibers with a mode dispersion of: 
EQU t.sub.F &gt;1 ns, 
interference is suppressed even though only short fiber lengths are 
required.

SPECIFIC DESCRIPTION 
The measuring device M1 shown diagrammatically in dot-dash lines in FIG. 1 
is provided with an optical transimitter formed by a light-emitting diode 
(LED) 1. The light 2 from the LED 1 passes through an internal light 
waveguide (LWL) 3 which can be a single-mode fiber (SMF) terminating in a 
plug connector. The internal light waveguide 3 can also be a multimode 
fiber (MMF) if desired. 
At the output of the measuring device M1, a plug connector 4 is provided in 
which the internal light waveguide 3 is optically coupled to an external 
light waveguide 5 which is shown as a single fiber. The coupling or 
connector 4 and the light waveguides 3 and 5 thus together form an optical 
plug connection. 
The ends 6 and 7 of the two light waveguides 3 and 5 are spaced apart by 
distance d to form an air gap between them, this distance d being 
sufficiently large as to preclude the formation of interference in the 
region between the ends 6 and 7. 
More particularly, the distance d is made larger than the coherence length 
l.sub.c of the light 2 emitted by the diode 1. The coherence length 
l.sub.c is given by the equation l.sub.c =.lambda..sub.0.sup.2 
/.DELTA..lambda., where .lambda..sub.0 is the centeral wavelength and 
.DELTA..lambda. is the spectral halfwidth of the light generated by the 
light-emitting diode. 
In the embodiment of FIG. 2, the measuring unit 2 is also schematically 
shown but has, as its light transmitter or source, a laser 9. The light 10 
transmitted from the laser 9 is fed to a multimode fiber 11. 
In this embodiment as well, where the internal light waveguide 11 is formed 
as a multimode fiber, the fiber ends 6 and 7 are spaced apart by an air 
gap in the plug connector 4. The plug connector 4, togehter with the light 
waveguides 5 and 11, form an optical plug connection. The transit time 
differences between the individual modes traversing the multimode fiber 
11, by a corresponding choice of the multimode fiber, is greater than the 
coherence time of the laser, thereby practically eliminating the 
possibility of interference phenomenon in the region of the air gap 8. An 
interference process thus cannot occur in the transmission from the 
internal waveguide 11 to the external waveguide 5. 
In FIG. 3, I have shown a plug connector 4 which can be of the type 
described, for example, in connection with FIG. 1 but which is shown in 
greatly enlarged scale. 
The details of the plug connector 4, however, have not been shown. What is 
important is that the fiber ends 6 and 7 are held apart by the distance d 
between the ends 6 and 7. This is done by grinding down one or both of the 
ends 6 and 7 and polish it or them by an amount sufficient to form the 
distance d when a commercial plug, which would normally causes the ends 6 
and 7 to bear upon one another, is used. In this case, the fiber is set 
back with respect to the part of the pin or sheath as it is inserted into 
the plug connector to ensure the proper spacing 8.